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

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

An earlier commit added support for unmasked scalar operations. At that time isel wouldn't generate an optimal sequence for masked operations, but that has now been fixed.

llvm-svn: 290566