Allow hoisting code in if-conversion.

When doing if-conversion, we do not currently move code out of the side
nodes.  The reason for this is that it can increase the number of
instructions that get executed because both side nods will have to be
executed now.

In this commit, we add code to move an instruction, and all of the
instructions it depends on, out of a side node and into the header of
the selection construct.  However to keep the cost down, we only do it
when the two values in the OpPhi node compute the same value.  This way
we have to move only one of the instructions and the other becomes
unused most of the time.  So no real extra cost.

Makes the value number table an alalysis in the ir context.

Added more opcodes to list of code motion safe opcodes.

Fixes #1526.