Clean out unneeded spill/reload instructions.

Handling of join points ~~~~~~~~~~~~~~~~~~~~~~~

B1: B2: ... ... RELOAD SLOT(0), %r1 RELOAD SLOT(0), %r1 ... A ... ... B ... jump B3 jump B3

B3: ... C ... RELOAD SLOT(0), %r1 ...

The Plan ~~~~~~~~ As long as %r1 hasn't been written to in A, B or C then we don't need the reload in B3.

What we really care about here is that on the entry to B3, %r1 will always have the same value that is in SLOT(0) (ie, %r1 is _valid_)

This also works if the reloads in B1/B2 were spills instead, because spilling %r1 to a slot makes that slot have the same value as %r1.