[DebugInfo][X86] Teach Optimize LEAs pass to handle debug values

This patch fixes an issue in the Optimize LEAs pass where redundant LEAs were
not removed because they were being used by debug values. The debug values are
now ignored when determining whether LEAs are redundant.

For now the debug values for the redundant LEAs are marked as undefined,
effectively lost. The intention is for a follow up patch which will attempt to
preserve the debug values where possible.

Patch by Andrew Ng.

Differential Revision: https://reviews.llvm.org/D30835

llvm-svn: 298360