gallivm: more integer texture format fetch fixes

gallivm: more integer texture format fetch fixes

Change the texel type to int/uint instead of float throughout the sampling
code which makes it easier to catch errors (as llvm will complain about wrong
types if we mistakenly treat these values as real floats somewhere).
This should also get things like e.g. sampler swizzles (for unused channels)
right.
This fixes piglit texture_integer_glsl130 test.
Border color not working (crashing) yet.
(These formats are not exposed yet in llvmpipe.)

v2: couple cleanups according to José's comments

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