[InstCombine] restrict select of bit-tests to constant shift amounts

This transform is responsible for a long-standing miscompile
as discussed in issue #47012 (was bugzilla #47668).

There was a proposal to correct it in D88432, but that was
abandoned and there hasn't been any recent activity to fix
it AFAICT.

The original patch D45108 started with a constant-shift-only
restriction and only expanded during review, so I don't think
there's much risk of perf regression on the motivating code.