[ARM] Correctly handle execute-only in EmitStructByval

Currently when compiling for an execute-only target without movt then
EmitStructByval will generate a constant pool load which isn't
compatible with execute-only. Handle this by emitting tMOVi32imm,
and also simplify the existing movt handling by emitting t2MOVi32imm
or MOVi32imm.

Differential Revision: https://reviews.llvm.org/D154944