[X86] Add a test case for r208252.
Prior to r208252, the FMA 231 family was marked as isCommutable. However the
memory variants of this family are not commutable. Therefore, we did not
implemented the findCommutedOpIndices for those variants and missed that
the default implementation (more or less: commute indices 1 and 2) was
firing behind our back.
As a result, as demonstrated in the test case before the fix, we were
transforming a = b * c + a into a = a * c + b.

I.e., before r208252 we were generating for this test case:
vmovaps %xmm0, %xmm1
vmoss (%rsi), %xmm0
vfmadd231ss (%rdi), %xmm1, %xmm0

Instead of:
vmoss (%rsi), %xmm1
vfmadd231ss (%rdi), %xmm1, %xmm0

<rdar://problem/16800495>
LTO: Check local linkage first

Since visibility is meaningless for symbols with local linkage, check
local linkage before visibility when setting symbol attributes.

When linkage is `internal` and the visibility is `hidden`, the exposed
attribute is now `LTO_SYMBOL_SCOPE_INTERNAL` instead of
`LTO_SYMBOL_SCOPE_HIDDEN`.  Although the bitfield allows *both* to be
specified, the combination is nonsense anyway.

Given changes (in progress) to drop visibility when a symbol has local
linkage, this almost has no functionality change: it's mostly a cleanup
to clarify the logic.

The exception is when something has `appending` linkage.  Before this
change, such symbols would be advertised as `LTO_SYMBOL_SCOPE_INTERNAL`;
now, they'll be given `LTO_SYMBOL_SCOPE_COMMON`.

Unfortunately this is really awkward to test.  This only changes what we
advertise to linkers (before running LTO), not what the final object
looks like.  In theory I could add `DEBUG` output to `llvm-lto` (and
test with "REQUIRES: asserts"), but follow-up commits to disallow
`internal hidden` simplify this anyway.

<rdar://problem/16141113>