[ARM] Avoid using ARM instructions in Thumb mode

The Requires class overrides the target requirements of an instruction,
rather than adding to them, so all ARM instructions need to include the
IsARM predicate when they have overwitten requirements.

This caused the swp and swpb instructions to be allowed in thumb mode
assembly, and the ARM encoding of CDP to be selected in codegen (which
is different for conditional instructions).

Differential Revision: https://reviews.llvm.org/D29283

llvm-svn: 293634