Minor DXIL compile warnings

[fstrugar]

Hi, thanks for a really great library! I stumbled upon a couple of compile warnings when compiling with the latest DXIL:

FFX_ParallelSort.h(266,13): error: redefinition of 'i' shadows declaration in the outer scope; most recent declaration will be used [-Werror,-Wfor-redefinition]

and

FFX_ParallelSort.h(381,31): error: ambiguous type for bit shift; use a type suffix on literal values, like 'L' or 'U', or a cast [-Werror,-Wambig-lit-shift]

Suggested fixes are in the #1

All the best