PPC NCG: Improve pointer de-tagging code
Generate a clrr[wd]i instruction to clear the tag bits in a pointer.
This saves one instruction and one temporary register.
Optimize signed comparison with zero after andi. operation This saves
one instruction when comparing a pointer tag with zero.
This reduces code size by 0.6 % in all nofib benchmarks.
Test Plan: validate on AIX and 32-bit Linux
Reviewed By: erikd, hvr
Differential Revision: https://phabricator.haskell.org/D2093