[x86] avoid crashing when splitting AVX stores with non-simple type
(PR43916)
The store splitting transform was assuming a simple type (MVT), but
that's not necessarily the case as shown in the test.
This should fix 'Assertion failed: (isSimple() && "Expected a
SimpleValueType!")' when building the security/openssl111 port targeting
a CPU that supports AVX, but not AVX2, such as sandybridge.
PR: 241747
MFC after: 1 month
X-MFC-With: r353358
699c9e3a49