Fix usage of APInt.getRawData for big-endian systems

Recommit modified version of r266311 including build bot regression fix.

This differs from the original r266311 by:

- Fixing Scalar::Promote to correctly zero- or sign-extend value depending
  on signedness of the *source* type, not the target type.

- Omitting a few stand-alone fixes that were already committed separately.

llvm-svn: 266422