teach tblgen to allow patterns like (add (i32 (bitconvert (i32 GPR))), 4),

transforming it into (add (i32 GPR), 4).  This allows us to write type
generic multi patterns and have tblgen automatically drop the bitconvert
in the case when the types align.  This allows us to fold an extra load
in the changed testcase.

llvm-svn: 99756