util/bitset: fix bitset range mask calculations.

util/bitset: fix bitset range mask calculations.

The MASK macro is used in the RANGE macro, and it should
return the pre-bitset word mask for the (b) value.

i.e.
BITSET_MASK(0) should be undefined since it's meaningless.
BITSET_MASK(31) should give 0x7fffffff
BITSET_MASK(32) should give 0xffffffff
BITSET_MASK(33) should give 0x00000001
BITSET_MASK(64) should give 0xffffffff

However then BITSET_RANGE ends up broken for cases where
it's (b) value is the 0,32,64 value as in that case the lower
mask would be 0 not 0xffffffff.

This fixes the unit tests that I've added, and my code that
uses bitsets.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Fixes: bb38cadb1c5f2 "More GLSL code"
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>