[DebugInfo][X86] Improve X86 Optimize LEAs handling of debug values.

This is a follow up to the fix in r298360 to improve the handling of debug
values when redundant LEAs are removed. The fix in r298360 effectively
discarded the debug values. This patch now attempts to preserve the debug
values by using the DWARF DW_OP_stack_value operation via prependDIExpr.

Moved functions appendOffset and prependDIExpr from Local.cpp to
DebugInfoMetadata.cpp and made them available as static member functions of
DIExpression.

Differential Revision: https://reviews.llvm.org/D31604

llvm-svn: 301630