A good trick for implementing lowering for a new target is, for not-yet-implemented instructions, to insert a FakeUse of each instruction variable followed by a FakeDef of the dest variable.  Otherwise one risks running afoul of liveness analysis integrity checks.

However, if all the high-level instructions in a basic block lack variables (e.g. unconditional branches, or void calls with only constant arguments), the resulting block may be completely empty.  In O2 mode, this triggers a couple of assertions/errors that wouldn't normally occur:

1. CfgNode::contractIfEmpty() finds a block with a single out-edge that does *not* end with an unconditional branch.

2. CfgNode::livenessAddIntervals() tries to add a bogus liveness interval to a variable because the empty block contains no actual instruction numbers to form a valid interval from.

This adds some fixes/workarounds for those problems.

Another workaround for the empty basic block problem may be to just to add a FakeUse of the stack pointer when lowering an unconditional branch, which combined with the trick above, should prevent empty blocks.  However, these fixes seem reasonable apart from that.

BUG= none
R=sehr@chromium.org

Review URL: https://codereview.chromium.org/1590303002 .