* x64: Add non-SSE4.1 lowerings of `pmov{s,z}x*`
This commit adds lowerings for a suite of sign/zero extension
instructions which don't require SSE4.1. Like before these lowerings are
based on LLVM's output.
This commit also deletes special casees for `i16x8.extmul_{low,high}_*`
since the output of the special case is the same as the default lowering
of all the component instructions used within as well.
* Remove SSE4.1 specialization of `uwiden_high`
LLVM prefers the `punpckh*`-based lowerings and at least according to
`llvm-mca` these are slightly better cycle-wise too.
edae6c0217