Remove the restriction on the first operand of the add node in SelectAddr.

This change reduces the number of instructions generated.

For example, 
(load (add (sub $n0, $n1), (MipsLo got(s))))

results in the following sequence of instructions:
1. sub $n2, $n0, $n1
2. lw got(s)($n2)

Previously, three instructions were needed.
1. sub $n2, $n0, $n1
2. addiu $n3, $n2, got(s)
3. lw 0($n3)

llvm-svn: 146888