[InstCombine] Avoid use of ConstantExpr::getZExtOrBitcast() (NFC)

Use the constant folding API instead. In the second case using
IR builder should also work, but the way the instructions are
created an inserted there is very unusual, so I've left it alone.

GitOrigin-RevId: b4afade17564c61e9baf8024f8cf6921bdb3bde3
2 files changed