[AVX-512] Add support for changing the element size of PALIGNR/VALIGND/VALIGNQ...

[AVX-512] Add support for changing the element size of PALIGNR/VALIGND/VALIGNQ shuffles if they feed a vselect with a different type

Summary:
Shuffle lowering widens the element size of a shuffle if elements are contiguous. This is sometimes help because wider element types have more shuffle options. If the shuffle is one of the arguments to a vselect this shuffle widening can introduce a bitcast between the vselect and the shuffle. This will prevent isel from selecting a masked operation. If the shuffle can be written equally efficiently with a different element size to match the vselect type we should change the shuffle type to allow masking.

This patch does this conversion for all VALIGND/VALIGNQ sizes. It also supports turning 128-bit PALIGNR into VALIGND/VALIGNQ. This fixes the case shown in PR31018.

I plan to add support for more operations in future patches.

Reviewers: RKSimon, zvi, delena

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26902

llvm-svn: 287612