[Thumb] Emit Thumb move in both Thumb modes for struct_byval predicates

Summary:
The MOV/MOVT instructions being chosen for struct_byval predicates was
conditional only on Thumb2, resulting in an ARM MOV/MOVT instruction
being incorrectly emitted in Thumb1 mode. This is especially apparent
with v8-m.base targets. This patch ensures that Thumb instructions are
emitted in both Thumb modes.

Reviewers: rengolin, t.p.northover

Subscribers: llvm-commits, aemerson, rengolin

Differential Revision: https://reviews.llvm.org/D22865

llvm-svn: 277128