IR: Thread OnlyIfReduced through ConstantExpr::getWithOperands()

In order to change `ConstantExpr::replaceUsesOfWithOnConstant()` to work
like other constants (e.g., using `ConstantArray::getImpl()`), thread
`OnlyIfReduced` through as necessary.  When `OnlyIfReduced` is false,
there's no functionality change.  When it's true, if there's no constant
folding or type changes `nullptr` is returned instead of the new
constant.

`ConstantExpr::replaceUsesOfWithOnConstant()` will be updated to use the
"true" version in a follow-up commit.

llvm-svn: 216015