[ARM] ldr pc,=expression should be allowed in Thumb2

This change allows the pc to be used as a destination register for the
pseudo instruction LDR pc,=expression . The pseudo instruction must not be
transformed into a MOV, but it can use the Thumb2 LDR (literal) instruction
to a constant pool entry. See (A7.7.43 from ARMv7M ARM ARM).

Differential Revision: https://reviews.llvm.org/D34751

llvm-svn: 307640