[DebugInfo][NFC] Early-exit when analyzing for single-location variables

This is a performance patch that hoists two conditions in DwarfDebug's
validThroughout to avoid a linear-scan of all instructions in a block. We
now exit early if validThrougout will never return true for the variable
location.

The first added clause filters for the two circumstances where
validThroughout will return true. The second added clause should be
identical to the one that's deleted from after the linear-scan.

Differential Revision: https://reviews.llvm.org/D77639