tgsi: obey clarified shift behavior

tgsi: obey clarified shift behavior

c shifts are undefined for shift counts exceeding (or matching) bit width,
so need to apply a mask (on x86 it actually would usually probably work as
shifts do masking on int domain shifts - unless some auto-vectorizer would
come along at last as simd domain does not mask the shift count).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>