[opt] Add -p alias for -passes

[aeubanks]

Summary

See [1] for background.

Some people have complained that opt -passes=instcombine is a lot more typing than opt -instcombine.
As a compromise that nobody has objected to in [1], allow opt -p instcombine.

[1] https://discourse.llvm.org/t/rfc-legacy-opt-pass-syntax-with-new-pass-manager/65863

[bjope]

LGTM

(Only concern is that -p used to mean something else. So we are re-introducing a short-option for something new. But I doubt that it will cause much confusion.)

llvm/docs/NewPassManager.rst

 This will be removed once the legacy pass manager is deprecated and removed for
 the optimization pipeline.
 
 To use the new PM:
 
 .. code-block:: shell
 
   $ opt -passes='pass1,pass2' /tmp/a.ll -S
+  # -p is an alias for -passes
+  $ opt -p pass1,pass2 /tmp/a.ll -S
 
 The new PM typically requires explicit pass nesting. For example, to run a
 function pass, then a module pass, we need to wrap the function pass in a module
 adaptor:
 
 .. code-block:: shell
 
   $ opt -passes='function(no-op-function),no-op-module' /tmp/a.ll -S

llvm/test/Other/new-pass-manager.ll

 ; This test is essentially doing very basic things with the opt tool and the
 ; new pass manager pipeline. It will be used to flesh out the feature
 ; completeness of the opt tool when the new pass manager is engaged. The tests
 ; may not be useful once it becomes the default or may get spread out into other
 ; files, but for now this is just going to step the new process through its
 ; paces.
 
 ; RUN: opt -disable-output -disable-verify -verify-cfg-preserved=1 -debug-pass-manager \
 ; RUN:     -passes=no-op-module %s 2>&1 \
 ; RUN:     | FileCheck %s --check-prefix=CHECK-MODULE-PASS
+; RUN: opt -disable-output -disable-verify -verify-cfg-preserved=1 -debug-pass-manager \
+; RUN:     -p no-op-module %s 2>&1 \
+; RUN:     | FileCheck %s --check-prefix=CHECK-MODULE-PASS
 ; CHECK-MODULE-PASS: Running pass: NoOpModulePass
 
 ; RUN: opt -disable-output -disable-verify -verify-cfg-preserved=1 -debug-pass-manager \
 ; RUN:     -passes=no-op-cgscc %s 2>&1 \
 ; RUN:     | FileCheck %s --check-prefix=CHECK-CGSCC-PASS
 ; RUN: opt -disable-output -disable-verify -verify-cfg-preserved=1 -debug-pass-manager \
 ; RUN:     -passes='cgscc(no-op-cgscc)' %s 2>&1 \
 ; RUN:     | FileCheck %s --check-prefix=CHECK-CGSCC-PASS

llvm/tools/opt/opt.cpp

 // to run over the module. This flag switches opt to use the new pass manager
 // infrastructure, completely disabling all of the flags specific to the old
 // pass management.
 static cl::opt<std::string> PassPipeline(
     "passes",
     cl::desc(
         "A textual description of the pass pipeline. To have analysis passes "
         "available before a certain pass, add 'require<foo-analysis>'."));
+static cl::alias PassPipeline2("p", cl::aliasopt(PassPipeline),
+                                   cl::desc("Alias for -passes"));
 
 static cl::opt<bool> PrintPasses("print-passes",
                                  cl::desc("Print available passes that can be "
                                           "specified in -passes=foo and exit"));
 
 static cl::opt<std::string>
 InputFilename(cl::Positional, cl::desc("<input bitcode file>"),
     cl::init("-"), cl::value_desc("filename"));