add X86 load folding tests for unary math ops

X86 load folding is fragile; eg, the tests here
don't work without AVX even though they should. This
is because we have a mix of tablegen patterns that have
been added over time, and we have a load folding table
used by the peephole optimizer that has to be kept in 
sync with the ever-changing ISA and tablegen defs.

llvm-svn: 229870