[X86] Don't alter HasOpaqueSPAdjustment after we've relied on it

We rely on HasOpaqueSPAdjustment not changing after we've calculated
things based on it.  Things like whether or not we can use 'rep;movs' to
copy bytes around, that sort of thing.  If it changes, invariants in the
backend will quietly break.  This situation arose when we had a call to
memcpy *and* a COPY of the FLAGS register where we would attempt to
reference local variables using %esi, a register that was clobbered by
the 'rep;movs'.

This fixes PR26124.

llvm-svn: 257730