Almost NFC (see end).
The backwards scan in validThroughout significantly contributed to compile time
for a pathological case, causing the 'X86 Assembly Printer' pass to account for
roughly 70% of the run time. This patch guards the loop against running
unnecessarily for a common case, bringing the pass contribution down to 4%.
There is a hack in validThroughout which promotes single constant value
DBG_VALUEs in the prologue to be live throughout the function. We're more
likely to hit this code path with this patch applied. Similarly to the parent patches
there is a small coverage change reported in the order of 10s of bytes.