[AMDGPU] fcanonicalize elimination optimization

We are using multiplication by 1.0 to flush denormals and quiet sNaNs.
That is possible to omit this multiplication if source of the
fcanonicalize instruction is known to be flushed/quieted, i.e.
if it comes from another instruction known to do the normalization
and we are using IEEE mode to quiet sNaNs.

Differential Revision: https://reviews.llvm.org/D35218

llvm-svn: 307848