ABI handling: Fix nasty thinko where IRgen could generate an out-of-bounds read

when generating a coercion for ABI handling purposes.
 - This may only manifest itself when building at -O0, but the practical effect
   is that other arguments may get clobbered.

 - <rdar://problem/6930451> [irgen] ABI coercion clobbers other arguments

llvm-svn: 72932