llvmpipe: use alpha from already converted color if possible

llvmpipe: use alpha from already converted color if possible

For rgbx formats, there is no point in doing alpha conversion again (and
with different tranpose even, so llvm can't eliminate it).
Albeit it looks like there's some minimal changes needed in the blend code
(found by code inspection, no test seemed to complain) if we do this -
the blend factors are already sanitized if we have no destination alpha,
however for src_alpha_saturate it looks like it still might make a
difference (note that we forced has_alpha to true before for some formats
and nothing complained, but this seems safer).

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