ARM: change implicit immediate forms of {ld,st}r{,b}t to psuedo-instructions

The implicit immediate 0 forms are assembly aliases, not distinct instruction
encodings.  Fix the initial implementation introduced in r198914 to an alias to
avoid two separate instruction definitions for the same encoding.

An InstAlias is insufficient in this case as the necessary due to the need to
add a new additional operand for the implicit zero.  By using the AsmPsuedoInst,
fall back to the C++ code to transform the instruction to the equivalent
_POST_IMM form, inserting the additional implicit immediate 0.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199032 91177308-0d34-0410-b5e6-96231b3b80d8