[AVX512] Correct isel patterns to support selecting masked vbroadcastf32x2/vbroadcasti32x2

Summary:
This patch adjusts the patterns to make the result type of the broadcast node vXf64/vXi64. Then adds a bitcast to vXi32 after that. Intrinsic lowering was also adjusted to generate this new pattern.

Fixes PR34357

We should probably just drop the intrinsic entirely and use native IR, but I'll leave that for a future patch.

Any idea what instruction we should be lowering the floating point 128-bit result version of this pattern to?  There's a 128-bit v2i32 integer broadcast but not an fp one.

Reviewers: aymanmus, zvi, igorb

Reviewed By: aymanmus

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37286

llvm-svn: 312101