[AMDGPU] Asm/disasm clamp modifier on vop3 int arithmetic

Allow the clamp modifier on vop3 int arithmetic instructions in assembly
and disassembly.

This involved adding a clamp operand to the affected instructions in MIR
and MC, and thus having to fix up several places in codegen and MIR
tests.

Differential Revision: https://reviews.llvm.org/D59267

Change-Id: Ic7775105f02a985b668fa658a0cd7837846a534e
llvm-svn: 356399