Avoid removing useful loop metadata when stripping debug info

The stripDebugLocFromLoopID() may mistakenly remove useful metadata nodes
when they are operands of a child node, which also has DILocation operands.

It can be reproduced by the output of clang for code similar to the following:

for(int i = 0; i < n; i++)
  x[i] = 10;
-strip-debug removes the child node of llvm.loop.vectorize.followup_all,
which contains llvm.loop.isvectorized and llvm.loop.unroll.count.

This patch fixes by checking all children nodes and only remove a metadata
node if all its children are DILocation.

Differential Revision: https://reviews.llvm.org/D141909