[LegalizeTypes] Teach PromoteIntRes_BITCAST to better handle a bitcast with...

[LegalizeTypes] Teach PromoteIntRes_BITCAST to better handle a bitcast with vector output type and a vector input type that needs to be widened

Summary: Previously if we had a bitcast vector output type that needs promotion and a vector input type that needs widening we would just do a stack store and load to handle the conversion. We can do a little better if we can widen the bitcast to a legal vector type the same size as the widened input type. Then we can do the bitcast between this widened type and the widened input type. Afterwards we can extract_subvector back to the original output and any_extend that. Type legalization will then circle back and handle promotion of the extract_subvector and the any_extend will just be removed. This will avoid going through the stack and allows us to remove a custom version of this legalization from X86.

Reviewers: efriedma, RKSimon

Reviewed By: efriedma

Subscribers: javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D53229

llvm-svn: 345567