[AIX][XCOFF] make integrated-as as default on AIX.

[brad]

Summary

IAS was enabled on AIX in Clang. Also make sure to follow up with the part of the diff (https://reviews.llvm.org/D150758) that was removed to enable support in the backend as well.

[shchenz]

llvm/lib/MC/MCAsmInfoXCOFF.cpp

55

Can we add a case to test that now llc on AIX by default will generate object directly? Like:

llc t.ll -filetype=obj -o t1.o
llc t.ll -o t2.o
diff t1.o t2.o

[nemanjai]

llvm/lib/MC/MCAsmInfoXCOFF.cpp

55

Is this actually what is expected? We use the integrated assembler on Linux and llc doesn't produce object files by default (nor would I want it to).

[shchenz]

llvm/lib/MC/MCAsmInfoXCOFF.cpp

55

You are right, Nemanja. This change will not impact the type of the output.

Seems it is hard(maybe impossible?) to construct a case reflecting this code change.
The only place that cause functionality change on AIX seems be in AsmPrinter::emitInlineAsm(). But MCAI->parseInlineAsmUsingAsmParser() is always true on AIX and there is no llc option can control it. So the change of MCAI->useIntegratedAssembler() will not cause difference on AIX.

[brad]

llvm/lib/MC/MCAsmInfoXCOFF.cpp

55

Seems it is hard(maybe impossible?) to construct a case reflecting this code change.

When I prodded a handful of backends like MSP430, VE, Hexagon to flip this option there were no cases adjusted or added. Just in some cases some issues with parsing some ASM tests

[shchenz]

LGTM

llvm/lib/MC/MCAsmInfoXCOFF.cpp

MCAsmInfoXCOFF::MCAsmInfoXCOFF() {
   // Use .vbyte for data definition to avoid directives that apply an implicit
   // alignment.
   Data16bitsDirective = "\t.vbyte\t2, ";
   Data32bitsDirective = "\t.vbyte\t4, ";
 
   COMMDirectiveAlignmentIsInBytes = false;
   LCOMMDirectiveAlignmentType = LCOMM::Log2Alignment;
   HasDotTypeDotSizeDirective = false;
-  UseIntegratedAssembler = false;
   ParseInlineAsmUsingAsmParser = true;
   NeedsFunctionDescriptors = true;
 
   ExceptionsType = ExceptionHandling::AIX;
 }
 
 bool MCAsmInfoXCOFF::isAcceptableChar(char C) const {
   // QualName is allowed for a MCSymbolXCOFF, and