[ARM] Change a couple of instances of LiveRegs.contains to !LiveRegs.available

This changes a couple of calls to LiveRegs.contains to
!LiveRegs.available, one in Thumb1FrameLoweringInfo (which modifies a
test to look more correct to me, given r7 should be the frame pointer so
is not available), and another in the ARMLoadStoreOptimizer, that I
don't have a test for, it was just found by inspection.

Differential Revision: https://reviews.llvm.org/D107454