[AArch64] Make am_ldrlit an iPTR - not OtherVT - operand. NFC-ish.

am_ldrlit diverged from am_brcond in r207105, but kept the OtherVT
operand type.  It made sense for branch targets, as those are
represented as MVT::Other in SDAG.  But loads operate on pointers.

This shouldn't have an observable effect on any in-tree code, but helps
make the patterns consistent for external users.

llvm-svn: 295229