[IR] Merge createReplacementInstr into ConstantExpr::getAsInstruction

createReplacementInstr was a trivial wrapper around
ConstantExpr::getAsInstruction, which also inserted the new instruction
into a basic block. Implement this directly in getAsInstruction by
adding an InsertBefore parameter and change all callers to use it. NFC.

A follow-up patch will remove createReplacementInstr.

Differential Revision: https://reviews.llvm.org/D112791