Remove valueCoversEntireFragment asserts in ConvertDebugDeclareToDebugValue

This is a fixup for r334830 causing problems in polly-aosp buildbot.

Focus in r334830 was to fix a problem seen with
ConvertDebugDeclareToDebugValue involving store instructions.
It also added some asserts to find out of similar problems
existed for the ConvertDebugDeclareToDebugValue functions
involving load and phi instructions. One of those asserts seems
to blow in the polly-aosp buildbot, so I'll revert the asserts
while debugging.

llvm-svn: 335031