[X86] Add ISD nodes for masked truncate so we can properly represent when the...

[X86] Add ISD nodes for masked truncate so we can properly represent when the output has more elements than the input due to needing to be 128 bits.

We can't properly represent this with a vselect since the upper elements of the result are supposed to be zeroed regardless of the mask.

This also reuses the new nodes even when the result type fits in 128 bits if the input is q/d and the result is w/b since vselect w/b using k-register condition isn't legal without avx512bw. Currently we're doing this even when avx512bw is enabled, but I might change that.

This fixes some of PR34877

llvm-svn: 350985