[llvm] Use more explicit cast methods (NFC)

Instead of ConstantExpr::getCast() with a fixed opcode, use the
corresponding getXYZ methods instead. For the one place creating
a pointer bitcast drop it entirely, as this is redundant with
opaque pointers.

GitOrigin-RevId: 739c86df807748182e576e11697db6002738aafd
5 files changed