mesa: Create _mesa_bitcount_64() to replace i965's brw_count_bits()

mesa: Create _mesa_bitcount_64() to replace i965's brw_count_bits()

The i965 driver already had a function to count bits in a 64-bit uint
(brw_count_bits()), but it was buggy (it only counted the bottom 32
bits) and it was clumsy (it had a strange and broken fallback for
non-GCC-like compilers, which fortunately was never used).  Since Mesa
already has a _mesa_bitcount() function, it seems better to just
create a _mesa_bitcount_64() function rather than special-case this in
the i965 driver.

This patch creates the new _mesa_bitcount_64() function and rewrites
all of the old brw_count_bits() calls to refer to it.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>