Split up br_if and br_table into multiple IR codes

[fengb]

Right now the code translates all the branches to be the same. If we had multiple different codes, we might be able to exploit branch prediction a bit better.

Example:

i32.load 1;
br_if 1;
i32.load 2;
br_if 2;

These two branches don't have much in common, but the predictor is forced to use the same branch. If these were 2 different opcodes, we would have 2 different bodies so the branch predictor can do a better job isolating events.