[AMDGPU] Assembler: Simplify handling of optional operands

Prepare to support DPP encodings.

For DPP encodings, we want row_mask/bank_mask/bound_ctrl to be optional operands. However this means that when parsing instruction which has no mnemonic prefix, we cannot add both default values for VOP3 and for DPP optional operands to OperandVector - neither instructions would match. So add default values for optional operands to MCInst during conversion instead.

Mark more operands as IsOptional = 1 in .td files.
Do not add default values for optional operands to OperandVector in AMDGPUAsmParser.
Add default values for optional operands during conversion using new helper addOptionalImmOperand.
Change to cvtVOP3_2_mod to check instruction flag instead of presence of modifiers. In the future, cvtVOP3* functions can be combined into one.
Separate cvtFlat and cvtFlatAtomic.
Fix CNDMASK_B32 definition to have no modifiers.

Review: http://reviews.llvm.org/D17445

Reviewers: tstellarAMD
llvm-svn: 261742