`linalg.matmul` already has an attribute for casts, defaults to signed
but allowed unsigned, so the operation `linalg.matmul_unsigned` is
redundant. The generalization test has an example on how to lower to
unsigned matmul in linalg.
This is the first PR in a list of many that will simplify the linalg
operations by using similar attributes.
Ref:
https://discourse.llvm.org/t/rfc-transpose-attribute-for-linalg-matmul-operations/80092
80e61e3842