i965: Remove broken source type assertions from brw_alu3().

i965: Remove broken source type assertions from brw_alu3().

Commit 526ffdfc033ab01cf133cb7e8290c65d12ccc9be attempted to generalize
the source register type assertions to allow D and UD.  However, the
src1 and src2 assertions actually checked src0.type against D and UD due
to a copy and paste bug.

It also began setting the source and destination register types based on
dest.type, ignoring src0/src1/src2.type completely.  BFE and BFI2 may
actually pass mixed D/UD types and expect them to be ignored, which is
arguably a bit sloppy, but not too crazy either.

This patch simply removes the source register assertions as those values
aren't used anyway.  It also clarifies the comment above the block that
sets the register types.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>