[x86] Add a bunch of test cases where we have different shuffle patterns
in the high and low 128-bit lanes of a v8f32 vector.

No functionality change yet, but wanted to set up the baseline for my
next patch which will make these quite a bit better. =]
Disable most of the google-readability-casting check for non-C++ files (only
leave the "redundant cast to the same type" part).
[x86] Teach the new vector shuffle lowering how to cleverly lower single
input v8f32 shuffles which are not 128-bit lane crossing but have
different shuffle patterns in the low and high lanes. This removes most
of the extract/insert traffic that was unnecessary and is particularly
good at lowering cases where only one of the two lanes is shuffled at
all.

I've also added a collection of test cases with undef lanes because this
lowering is somewhat more sensitive to undef lanes than others.