[ARM] Correctly handle combining segmented stacks with execute-only

Using segmented stacks with execute-only mostly works, but we need to
use the correct movi32 opcode in 6-M, and there's one place where for
thumb1 (i.e. 6-M and 8-M.base) a constant pool was unconditionally
used which needed to be fixed.

Differential Revision: https://reviews.llvm.org/D156339