[update_llc_test_checks] Use FileCheck captures for MCInst/MCReg output

To avoid test churn when backends add/rename new instructions/registers,
it makes sense to use FileCheck captures for the exact MCInst/Reg number.
This is motivated by D125307, where I use --asm-show-inst to differentiate
the output for multiple instructions with the same mnemonic.

This does not quite fix the churn issue yet: While files with the generated
checks will be immune to the numbers changing, the update script test
still suffers from this problem since the number is encoded in the
FileCheck variable name. I plan to address this in a follow-up patch.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D125307