[SDAG] move x86 select-with-identity-constant fold behind a target hook; NFC

This is no-functional-change-intended because only the
x86 target enables the TLI hook currently.

We can add fmul/fdiv opcodes to the switch similar to the
proposal D119111, but we don't need to make other changes
like enabling target-specific combines.

We can also add integer opcodes (add, or, shl, etc.) to
the switch because this function is called from all of the
generic binary opcodes.

The goal is to incrementally enable the profitable diffs
from D90113 while avoiding regressions.

Differential Revision: https://reviews.llvm.org/D119150

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