Bit-shift expressions: Cast scalar right-hand-sides to vectors if needed

[kinke]

This promotion from scalar right-hand-sides to vectors already happens for the arithmetic binops, via typeCombine() side effects. Handle bit-shift expressions too now, by casting a scalar to the left-hand-side vector type.

This can't be tested with DMD, as DMD's Target.isVectorOpSupported() doesn't support bit-shifts yet. It is ~required/useful for LDC though.

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