[ARM] Fix STRT/STRHT/STRBT input/output operands.

STRT, STRHT, and STRBT are store instructions and their source register
$Rt should be treated as an input operand instead of an output operand.
This should fix things (e.g., liveness tracking in LivePhysRegs) if
these instructions were used in CodeGen.

Differential Revision: https://reviews.llvm.org/D95074