[AArch64][GlobalISel] NFC: Replace IR regbankselect test with MIR test

regbank-ceil.ll -> regbank-ceil.mir

The IR test was intended to only check register banks. This makes it brittle,
especially as we improve load/store combines in GlobalISel.

Rewriting this as a MIR test also makes it more consistent with the rest of
the testcases in GlobalISel.