[SCEV] Avoid creating unnecessary SCEVs for SelectInsts.

After 675080a4533b, we always create SCEVs for all operands of a
SelectInst. This can cause notable compile-time regressions compared to
the recursive algorithm, which only evaluates the operands if the select
is in a form we can create a usable expression.

This approach adds additional logic to getOperandsToCreate to only
queue operands for selects if we will later be able to construct a
usable SCEV.

Unfortunately this introduces a bit of coupling between actual SCEV
construction for selects and getOperandsToCreate, but I am not sure if
there are better alternatives to address the regression mentioned for
675080a4533b.

This doesn't have any notable compile-time impact on CTMark.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D129731

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