gallivm: introduce 32x32->64bit lp_build_mul_32_lohi function

gallivm: introduce 32x32->64bit lp_build_mul_32_lohi function

This is used by shader umul_hi/imul_hi functions (and soon by draw).
It's actually useful separating this out on its own, however the real
reason for doing it is because we're using an optimized sse2 version,
since the code llvm generates is atrocious (since there's no widening
mul in llvm, and it does not recognize the widening mul pattern, so
it generates code for real 64x64->64bit mul, which the cpu can't do
natively, in contrast to 32x32->64bit mul which it could do).

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