[AVX-512][InstCombine] Teach InstCombine to turn scalar add/sub/mul/div with...

[AVX-512][InstCombine] Teach InstCombine to turn scalar add/sub/mul/div with rounding intrinsics into normal IR operations if the rounding mode is CUR_DIRECTION.

Summary:
I only do this for unmasked cases for now because isel is failing to fold the mask. I'll try to fix that soon.

I'll do the same thing for packed add/sub/mul/div in a future patch.

Reviewers: delena, RKSimon, zvi, craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D27879

llvm-svn: 290535