[TypePromotion] Query target register width

TargetLoweringInfo may report that an integer should be promoted, but
it maybe provide a size that isn't natively supported by the target
register file... So check this before trying to perform a promotion.

This is to fix some chromium issues:
https://bugs.chromium.org/p/chromium/issues/detail?id=1031978
https://bugs.chromium.org/p/chromium/issues/detail?id=1031979

Differential Revision: https://reviews.llvm.org/D71200