[PPCGCodeGen] Fix compiler warning: '<': signed/unsigned mismatch. NFC.

MSVC warns about comparison between a signed and unsigned integer.
The rules of C(++) define that an unsigned comparison has to be
carried-out in this case. This is unlikely to be intended.

Fix by assigning the loop's upper bound to a signed integer first.
This also avoids repeated evaluation of the invariant upper bound.

llvm-svn: 311548