Fixes two false positives in the drcachesim invariant checker code. Skips the pre-signal instr check when returning from signals that arrived before any other instr in the trace, or before any other instr since the last kernel xfer (nested signals without any intervening instr). In both these cases we don't have any pre-signal instr whose pc we can compare with the post-signal instr.
Note that we still do not handle the back-to-back signals case where the second signal arrives just after the first one returns, without any intervening instruction. Improved the comment for that.
Adds tests that fail without this fix.
Fixes: #5784 (closed)