[SystemZ] Introducing assembler dialects for the Z backend

[anirudhp]

Summary

• This patch introduces a different assembler dialect ("hlasm") for z/OS. The default dialect has now been given the "att" dialect name. For this appropriate changes have been added to SystemZ.td.
• This patch also makes a few changes to SystemZInstrFormats.td which restrict a few condition code mnemonics to just the "att" dialect variant (he, le, lh, nhe, nle, nlh). These extended condition code mnemonics are not available in HLASM.
• A new private function has been introduced in SystemZAsmParser.cpp to return the assembler dialect set in SystemZMCAsmInfo.cpp. The reason we couldn't/haven't explicitly queried the overriden getAssemblerDialect function from AsmParser is outlined in this thread here. This returned dialect is directly passed onto the relevant matcher functions which taken in a variantID, so that the matcher functions can appropriately choose an instruction based on the variant.

[anirudhp]

Ping :)

[uweigand]

Just a quick comment that I'm looking at this, but before approval I want to resolve the issues described in the comment in front of getMAIAssemblerDialect. See also discussion here: https://reviews.llvm.org/D82862

[anirudhp]

In D94250#2498246, @uweigand wrote:

Just a quick comment that I'm looking at this, but before approval I want to resolve the issues described in the comment in front of getMAIAssemblerDialect. See also discussion here: https://reviews.llvm.org/D82862

Thanks!