[fast-isel] Fold "urem x, pow2" -> "and x, pow2-1". This should fix the 271%

execution-time regression for nsieve-bits on the ARMv7 -O0 -g nightly tester.
This may also improve compile-time on architectures that would otherwise 
generate a libcall for urem (e.g., ARM) or fall back to the DAG selector.
rdar://10810716

llvm-svn: 153230