[NFC][Thumb1] Use FrameDestroy flag to identify epilog instructions

Simiarly to what's done on both ARM's and AArch64's frame lowering code,
this updates Thumb1FrameLowering to use the FrameDestroy Machine
Instruction flag to identify instructions inserted as part of the epilog
instead of relying on assumptions about specific machine instructions.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D126285