For PowerPC instruction "addi", use constraint "b".
Without "b", a variant of the tas() code miscompiles on macOS 10.4.
This may also fix a compilation failure involving macOS 10.1. Today's
compilers have been allocating acceptable registers with or without this
change, but this future-proofs the code by precisely conveying the
acceptable registers. Back-patch to 9.4 (all supported versions).
Reviewed by Tom Lane.
Discussion: https://postgr.es/m/
20191009063900.GA4066266@rfd.leadboat.com