[PATCH] optimize MachineBasicBlock::getSymbol a bit

[Eli Bendersky]

MachineBasicBlock::getSymbol gets called quite a lot and was doing a lot of
repetitive work. Most of the symbol remains unchanged between MBBs in the
same function, and yet getSymbol was laboriously reconstructing the full
symbol with a concatenation of Twines. This patch optimizes this away:
MachineFunction caches the symbol "template" for its MBBs and does a
minimal amount of work to generate a new symbol without too much copying.

There's no change in functionality, but this makes
MachineBasicBlock::getSymbol somewhere between 25-50% faster (exact numbers
are hard to obtain for this very internal method).

Please take a look,
Eli
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