[GVN] Partially revert debug info salvage change (r325063)

In r325063, we salvaged debug values from dying instructions in
GVN::processBlock() and GVN::performScalarPRE().

The change in performScalarPRE(), while correct, is unhelpful. It
introduced a call to salvageDebugInfo() which was immediately followed
by a RAUW, meaning it prevented the RAUW from efficiently updating
dbg.value intrinsics.  This commit reverts the mistake and tightens up
the affected test case.

llvm-svn: 325308