[x86] generalize reassociation optimization in machine combiner to 2 instructions

Currently ( D10321, http://reviews.llvm.org/rL239486 ), we can use the machine combiner pass

to reassociate the following sequence to reduce the critical path:

A = ? op ?

B = A op X

C = B op Y

-->

A = ? op ?

B = X op Y

C = A op B

'op' is currently limited to x86 AVX scalar FP adds (with fast-math on), but in theory, it could

be any associative math/logic op (see TODO in code comment).

This patch generalizes the pattern match to ignore the instruction that defines 'A'. So instead of

a sequence of 3 adds, we now only need to find 2 dependent adds and decide if it's worth

reassociating them.

This generalization has a compile-time cost because we can now match more instruction sequences

and we rely more heavily on the machine combiner to discard sequences where reassociation doesn't

improve the critical path.

For example, in the new test case:

A = M div N

B = A add X

C = B add Y

We'll match 2 reassociation patterns, but this transform doesn't reduce the critical path:

A = M div N

B = A add Y

C = B add X

We need the combiner to reject that pattern but select this:

A = M div N

B = X add Y

C = B add A

Differential Revision: http://reviews.llvm.org/D10460