[ARM] Delay ARM constant pool creation. NFC.

This change adds a new constant pool kind to ARMOperand. When parsing the
operand for =immediate we create an instance of this operand rather than
creating a constant pool entry and rewriting the operand.

As the new operand kind is only created for ldr rt,= we can make ldr rt,=
an explicit pseudo instruction in ARM, Thumb and Thumb2

The pseudo instruction is expanded in processInstruction(). This creates the
constant pool and transforms the pseudo instruction into a pc-relative ldr to
the constant pool.

There are no functional changes and no modifications needed to existing tests.

Required by the patch that fixes PR25722.

Patch by Peter Smith.

llvm-svn: 269352