R600: Fix inconsistency in rsq instructions.

R600 was using a clamped version of rsq, but SI was not. Add a
new rsq_clamped intrinsic and use them consistently.

It's unclear to me from the documentation what behavior
the R600 instructions have, so I assume they have the legacy behavior
described by the SI documents. For R600, use RECIPSQRT_IEEE
for both llvm.AMDGPU.rsq.legacy and llvm.AMDGPU.rsq. R600 also
has RECIPSQRT_FF, which I'm not sure how it fits in here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211637 91177308-0d34-0410-b5e6-96231b3b80d8