[InstCombine] add more insert-extract tests for D52439; NFC

The first attempt at this transform:
rL343407
...was reverted:
rL343458
...because it did not handle the case where we bitcast to FP. 
The patch was already limited to avoid the case where we
bitcast from FP, but we might want to transform that too.

llvm-svn: 343480