Remove PHIs and introduce block args

[vaivaswatha]

The Sway-IR currently limits basic blocks to have only a single PHI node. That wouldn't work, for example, in the following case:

let mut x;
let mut y;
if (...) {
    x = 10;
    y = 11;
} else {
    x = 11;
    y = 10;
}
merge_block:
     = x;
     = y;

At the merge block we'll need two PHIs, one for x and another for y.

We have two choices:

1. Remodel the IR to allow multiple PHIs and teach the various IR transformations / analyses about it.
2. Go for the "more modern" block argument approach.

Working with multiple PHIs is a bit clumsy, and in our case, since there is an assumption throughout the IR that there is only a single PHI, making changes to it to accept multiple PHIs has the risk of breaking this assumption somewhere. It's safer to just remove PHIs altogether and move to block arguments.