[stack protector] Make the StackProtector pass respect ssp-buffer-size.

Previously, SSPBufferSize was assigned the value of the "stack-protector-buffer-size"
attribute after all uses of SSPBufferSize.  The effect was that the default
SSPBufferSize was always used during analysis.  I moved the check for the
attribute before the analysis; now --param ssp-buffer-size= works correctly again.

Differential Revision: http://reviews.llvm.org/D3349