Elide repeated register operand in Thumb1 instructions

This patch makes the ARM backend transform 3 operand instructions such as
'adds/subs' to the 2 operand version of the same instruction if the first
two register operands are the same.

Example: 'adds r0, r0, #1' will is transformed to 'adds r0, #1'.

Currently for some instructions such as 'adds' if you try to assemble
'adds r0, r0, #8' for thumb v6m the assembler would throw an error message
because the immediate cannot be encoded using 3 bits.

The backend should be smart enough to transform the instruction to
'adds r0, #8', which allows for larger immediate constants.

Patch by Ranjeet Singh.