rs6000: MMA built-ins reject typedefs of MMA types

rs6000: MMA built-ins reject typedefs of MMA types

We do not allow conversions between the MMA types and other types.
However, we are being too strict in not matching MMA types with
typdefs of those types.  Use TYPE_CANONICAL to see through the
types to their canonical types before comparing them.

2020-08-08  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR target/96530
	* config/rs6000/rs6000.c (rs6000_invalid_conversion): Use canonical
	types for type comparisons.  Refactor code to simplify it.

gcc/testsuite/
	PR target/96530
	* gcc.target/powerpc/pr96530.c: New test.

(cherry picked from commit e2882e76089cecdc268d0835c54cabfa80b5b0be)