[AMDGPU] Support shared literals in FMAMK/FMAAK

These instructions should allow src0 to be a literal with the same
value as the mandatory other literal. Enable it by introducing an
operand that defers adding its value to the MI when decoding till
the mandatory literal is parsed.

Reviewed By: dp, foad

Differential Revision: https://reviews.llvm.org/D111067

Change-Id: I22b0ae0d35bad17b6f976808e48bffe9a6af70b7