[CGP] DisableBranchOpts guards against EliminateMostlyEmptyBlocks

[jingyue]

Summary

DisableBranchOpts is designed to disable all branch optimizations including
EliminateMostlyEmptyBlocks.

lib/CodeGen/CodeGenPrepare.cpp

if (!OptSize && TLI && TLI->isSlowDivBypassed()) {
     const DenseMap<unsigned int, unsigned int> &BypassWidths =
        TLI->getBypassSlowDivWidths();
     for (Function::iterator I = F.begin(); I != F.end(); I++)
       EverMadeChange |= bypassSlowDivision(F, I, BypassWidths);
   }
 
   // Eliminate blocks that contain only PHI nodes and an
   // unconditional branch.
+  if (!DisableBranchOpts)
     EverMadeChange |= EliminateMostlyEmptyBlocks(F);
 
   // llvm.dbg.value is far away from the value then iSel may not be able
   // handle it properly. iSel will drop llvm.dbg.value if it can not
   // find a node corresponding to the value.
   EverMadeChange |= PlaceDbgValues(F);
 
   // If there is a mask, compare against zero, and branch that can be combined
   // into a single target instruction, push the mask and compare into branch

test/Transforms/CodeGenPrepare/disable-branch-opts.ll

+; RUN: opt < %s -S -codegenprepare -disable-cgp-branch-opts | FileCheck %s
+
+define void @foo(i32 %x, float* %output) {
+; CHECK-LABEL: @foo(
+  %1 = icmp eq i32 %x, 1
+  br i1 %1, label %then, label %else
+
+then:
+; CHECK: then:
+  br label %merge
+
+else:
+; CHECK: else:
+  br label %merge
+
+merge:
+; CHECK: merge:
+  store float 2.0, float* %output
+  ret void
+}