[Constant Hoisting] Replace the MapVector with a separate Map and Vector to keep track of constant candidates.

This simplifies working with the constant candidates and removes the tight
coupling between the map and the vector.

Related to <rdar://problem/16381500>
[Constant Hoisting] Fix capitalization of function names.
[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>
[Constant Hoisting] Lazily compute the idom and cache the result.

Related to <rdar://problem/16381500>
[Constant Hoisting] Make the constant materialization cost operand dependent

Extend the target hook to take also the operand index into account when
calculating the cost of the constant materialization.

Related to <rdar://problem/16381500>