Enhance folding of (extract_subvec (insert_subvec V1, V2, IIdx), EIdx)

- Handle the case where the result of 'insert_subvect' is bitcasted
  before 'extract_subvec'. This removes the redundant insertf128/extractf128
  pair on unaligned 256-bit vector load/store on vectors of non 64-bit integer.

llvm-svn: 177945