[PowerPC] Add addtional test that retroactively catches PR47259

Due to the unfortunate way the bug could only be triggered when reading SPRG[0-3] into a register lower than %r4 with the "mfsprg %rX, 0" syntax, the tests did not detect it.

(It could not be triggered for "mfsprg0, %r2" because that pattern was already in the table, so the earlier "correct" match took effect)

As a canary, add an intentionally ambiguous "mfsprg 2, 2" and "mtsprg 2, 2" check that would have caught the problem.

Reviewed By: ZhangKang

Differential Revision: https://reviews.llvm.org/D86489