[InstSimplify] Simplify calls with "returned" attribute

If a call argument has the "returned" attribute, we can simplify
the call to the value of that argument. The "-inst-simplify" pass
already handled this for the constant integer argument case via
known bits, which is invoked in SimplifyInstruction. However,
non-constant (or non-int) arguments are not handled at all right now.

This addresses one of the regressions from D75801.

Differential Revision: https://reviews.llvm.org/D75815