[SCCP] Use SimplifyBinOp for non-integer constant/expressions & overdef.

For non-integer constants/expressions and overdefined, I think we can
just use SimplifyBinOp to do common folds. By just passing a context
with the DL, SimplifyBinOp should not try to get additional information
from looking at definitions.

For overdefined values, it should be enough to just pass the original
operand.

Note: The comment before the `if (isconstant(V1State)...` was wrong
originally: isConstant() also matches integer ranges with a single
element. It is correct now.

Reviewers: efriedma, davide, mssimpso, aartbik

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D76459