[X86] Mark all byval parameters as aliased

This is a fix for PR30290: by marking all byval stack slots as being aliased,
the instruction scheduler is more conservative about rescheduling memory
accesses to such stack slots as an LLVM Value* might alias it. This fixes
errors such as in the patched test case, where reads and writes to a data
structure are illegally mixed.

This could be fixed better in the future with better analysis for the
instruction scheduler to know what Values alias what stack slots.

Differential Revision: https://reviews.llvm.org/D45022

llvm-svn: 331749