[EarlyCSE] Check if getValueType() returns nullptr (#195868)
15383dcdb4012387d94fab67fed4519613bf9a19 recently added a call to
getValueType() into EarlyCSE without checking whether or not it returned
nullptr. It sometimes might end up processing target specific intrinsics
for which the underlying getAccessType will typically return nullptr.
We observed this crash when building https://github.com/google/highway
for AArch64.
Regression test added to ensure this does not occur again in the future.