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.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177945 91177308-0d34-0410-b5e6-96231b3b80d8