[MC/AsmParser] Avoid setting MCSymbol.IsUsed in some cases

Avoid marking some MCSymbols as used in MC/AsmParser.cpp when no uses
exist. This fixes a bug in parseAssignmentExpression() which
inadvertently sets IsUsed, thereby triggering:

    "invalid re-assignment of non-absolute variable"

on otherwise valid code. No other functionality change intended.

The original version of this patch touched many calls to MCSymbol
accessors. On rafael's advice, I have stripped this patch down a bit.

As a follow-up, I intend to find the call sites which intentionally set
IsUsed and force them to do so explicitly.

Differential Revision: http://reviews.llvm.org/D12347

llvm-svn: 246457