Add ability to emit internal instruction representation to CodeGen assembly output.

[dsanders]

Summary

This patch re-uses the implementation of 'llvm-mc -show-inst' and makes it
available to llc as 'llc -asm-show-inst'.

This is necessary to test parts of MIPS32r6/MIPS64r6 without resorting to
'llc -filetype=obj' tests. For example, on MIPS32r2 and earlier we use the
'jr $rs' instruction for indirect branches and returns. On MIPS32r6, we no
longer have 'jr $rs' and use 'jalr $zero, $rs' instead. The catch is that,
on MIPS32r6, 'jr $rs' is an alias for 'jalr $zero, $rs' and is the preferred
way of writing this instruction. As a result, all MIPS ISA's emit 'jr $rs' in
their assembly output and the assembler encodes this to different opcodes
according to the ISA.

Using this option, we can check that the MCInst really is a JR or a JALR by
matching the emitted comment. This removes the need for a 'llc -filetype=obj'
test.

[dsanders]

ping ...

[zoran.jovanovic]

LGTM.

include/llvm/MC/MCTargetOptionsCommandFlags.h

 
 cl::opt<bool> RelaxAll("mc-relax-all",
                        cl::desc("When used with filetype=obj, "
                                 "relax all fixups in the emitted object file"));
 
 cl::opt<int> DwarfVersion("dwarf-version", cl::desc("Dwarf version"),
                           cl::init(0));
 
+cl::opt<bool> ShowMCInst("asm-show-inst",
+                         cl::desc("Emit internal instruction representation to "
+                                  "assembly file"));
+
 static inline MCTargetOptions InitMCTargetOptionsFromFlags() {
   MCTargetOptions Options;
   Options.SanitizeAddress =
       (AsmInstrumentation == MCTargetOptions::AsmInstrumentationAddress);
   Options.MCRelaxAll = RelaxAll;
   Options.DwarfVersion = DwarfVersion;
+  Options.ShowMCInst = ShowMCInst;
   return Options;
 }
 
 #endif