[X86] Make the type checks in detectAVX512USatPattern more robust

This code currently uses isSimple and getSizeInBits in an attempt to prune types. But isSimple will return true for any type that any target supports natively. I don't think that's a good way to prune types. I also don't think the dest element type checks are very robust since we didn't do an isSimple check on the dest type.

This patch adds a check for the input type being legal to the one caller that didn't already check that. Then we explicitly check the element types for the destination are i8, i16, or i32

Differential Revision: https://reviews.llvm.org/D42706

llvm-svn: 323924