gallium: clarify shift behavior with shift count >= 32

Previously, nothing was said what happens with shift counts exceeding
bit width of the values to shift. In theory 3 behaviors are possible:
1) undefined (classic c definition)
2) just shift out all bits (so result is zero, or -1 potentially for ashr)
3) mask the shift count to bit width - 1
API's either require 3) or are ok with 1). In particular, GLSL (as well as a
couple uninteresting legacy GL extensions) is happy with undefined, whereas
both OpenCL and d3d10 require 3). Consequently, most hw also implements 3).
So, for simplicity we just specify that 3) is required rather than saying
undefined and then needing state trackers to work around it.
Also while here specify shift count as a vector, not scalar. As far as I
can tell this was a doc bug, neither state trackers nor drivers used scalar
shift count.

Reviewed-by: Jose Fonseca <[email protected]>

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