IR: Reduce RAUW traffic in ConstantExpr

Avoid RAUW-ing `ConstantExpr` when an operand changes unless the new
`ConstantExpr` already has users.  This prevents the RAUW from rippling
up the expression tree unnecessarily.

This commit indirectly adds test coverage for r215953 (this is how I
came across the bug).

This is part of PR20515.

llvm-svn: 215960