[AMDGPU] Ressociate 'add (add x, y), z' to use SALU

Reassociate adds to collect scalar operands in a single
instruction when possible. That will result in a scalar
add followed by vector instead of two vector adds, thus
better utilizing SALU.

Differential Revision: https://reviews.llvm.org/D58220

llvm-svn: 354066