Change all stackops to use only allocas

[ebo, 2010-08-21 18:59:19]

This patch changes all stack operations to use allocas instead of writing
to/reading from memory.
The stack is only written to memory if needed, which means before bailing and
exception handling and during some opcode implementations.

This gives a slight speed increase on all benchmarks (between 1.01 and 1.10
times faster) and will enable other more advanced optimizations.

[Reid Kleckner, 2010-08-29 15:54:28]

Sorry for the delay.  Any perf results?

http://codereview.appspot.com/1994044/diff/1/8
File JIT/llvm_fbuilder.cc (right):

http://codereview.appspot.com/1994044/diff/1/8#newcode1284
JIT/llvm_fbuilder.cc:1284: Value *former_top =
this->builder_.CreateLoad(this->stack_[this->stack_top_]);
80

http://codereview.appspot.com/1994044/diff/1/6
File JIT/opcodes/block.cc (right):

http://codereview.appspot.com/1994044/diff/1/6#newcode231
JIT/opcodes/block.cc:231: this->fbuilder_->SaveAllToStack();
Doesn't the unwind block save things to the stack?  Comment why the SaveAll is
needed, I can't actually tell.

http://codereview.appspot.com/1994044/diff/1/4
File JIT/opcodes/control.cc (right):

http://codereview.appspot.com/1994044/diff/1/4#newcode145
JIT/opcodes/control.cc:145: this->fbuilder_->SaveAllToStack();
Return also goes to the unwind block, which saves the stack.  I'd comment why
this is needed if it is.

http://codereview.appspot.com/1994044/diff/1/4#newcode169
JIT/opcodes/control.cc:169: this->fbuilder_->SaveAllToStack();
ditto, comment below notwithstanding.  We shouldn't be using anything past the
SP.