AMDGPU: Reduce code size with fcanonicalize (fneg x)

When fcanonicalize is lowered to a mul, we can
use -1.0 for free and avoid the cost of the bigger
encoding for source modifers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@338244 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed