Use an EncCursor instead of a layout cursor to keep track of the
current position in the function. Since the EncCursor holds a reference
to the whole IR function insteadof just the layout, we can rework how IR
borrowing works.
The Context data structure that's live during the spilling pass now owns
an EncCursor which in turn holds references to the function and ISA.
This means that we no longer need to pass around references to parts of
the ir::Function. We can no longer borrow any part of the IR function
across a context method call, but that turns out to be not necessary.
3eb80fde15