[ARM] Use rGPR for writeback vldrs

From what I can tell, a writeback is unpredictable with LR for both
loads and stores. This changes the operand from a gprnopc to a rGPR in
both cases (which I believe is essentially a NFC due to the tied-def
already being a rGPR.)

Differential Revision: https://reviews.llvm.org/D96723