[VectorUtils] add IR-level analysis for widening of shuffle mask

This is similar to the recent move/addition of "scaleShuffleMask" (D76508),
but there are a couple of differences:

1. The existing x86 helper (canWidenShuffleElements) always tries to
   divide-by-2, so it gets called iteratively and wouldn't handle the
   general case of non-pow-2 length.
2. The existing x86 code handles "SM_SentinelZero" - we don't have
   that in IR, but this code should be safe to use with that or other
   special (negative) values.

The motivation is to enable shuffle folds in instcombine/vector-combine
that are similar to D76844 and D76727, but in the reverse-bitcast direction.
Those patterns are visible in the tests for D40633.

Differential Revision: https://reviews.llvm.org/D77881

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