[InstCombine] canonicalize trunc + insert as bitcast + shuffle, part 2

This enhances the base fold from part 1 to allow mapping a
right-shift to an insert index.

Example of translating a middle chunk of the scalar to vector
for either endian:
https://alive2.llvm.org/ce/z/fRXCOZ

This only allows creating an identity shuffle (with optional
shortening/lengthening) because that is considered the safe
baseline for any target (can be inverted if needed). If we
tried this fold with target-specific costs/legality, then we
could do the transform more generally.

Differential Revision: https://reviews.llvm.org/D138873

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