[AMDGPU] Convert shl (add) into add (shl)

shl (or|add x, c2), c1 => or|add (shl x, c1), (c2 << c1)
This allows to fold a constant into an address in some cases as
well as to eliminate second shift if the expression is used as
an address and second shift is a result of a GEP.

Differential Revision: https://reviews.llvm.org/D33432

llvm-svn: 303641