draw,gallivm,llvmpipe: Avoid implicit casts of 32-bit shifts to 64-bits.

draw,gallivm,llvmpipe: Avoid implicit casts of 32-bit shifts to 64-bits.

Addresses MSVC warnings "result of 32-bit shift implicitly converted to
64 bits (was 64-bit shift intended?)", which can often be symptom of
bugs, but in these cases were all benign.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>