MC tweak symbol difference resolution for non-local symbols.

MC tweak symbol difference resolution for non-local symbols.

When the non-local symbol in the expression is in the same fragment
as the second symbol, the assembler can still evaluate the expression
without needing a relocation.

For example, on ARM:
_foo:
	ldr lr, (_foo - 4)

rdar://10348687

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148341 91177308-0d34-0410-b5e6-96231b3b80d8