[Constant Hoisting] Change the algorithm to only track constants for instructions.

Originally the algorithm would search for expensive constants and track their

users, which could be instructions and constant expressions. This change only

tracks the constants for instructions, but constant expressions are indirectly

covered too. If an operand is an constant expression, then we look through the

expression to find anny expensive constants.

The algorithm keep now track of the instruction and the operand index where the

constant is used. This allows more precise hoisting of constant materialization

code for PHI instructions, because we only hoist to the basic block of the

incoming operand. Before we had to find the idom of all PHI operands and hoist

the materialization code there.

This also makes updating of instructions easier. Before we had to keep track of

the original constant, find it in the instructions, and then replace it. Now we

can just simply update the operand.

Related to rdar://problem/16381500