[AMDGPU] Create new instructions in SIInstrInfo::moveToVALU

Create new VALU instructions in moveToVALU instead of mutating the
existing SALU instruction. This makes it easier to add extra operands so
we can convert to the VOP3 form of VALU instructions.

NFCI but it does have the minor side effect of removing duplicate
implicit operands that were present on the original SALU if they are
default implicit operands for the VALU.

Differential Revision: https://reviews.llvm.org/D137324