LoopVectorizer - skip 'bitcast' between GEP and load.

Skipping 'bitcast' in this case allows to vectorize load:

  %arrayidx = getelementptr inbounds double*, double** %in, i64 %indvars.iv
  %tmp53 = bitcast double** %arrayidx to i64*
  %tmp54 = load i64, i64* %tmp53, align 8

Differential Revision http://reviews.llvm.org/D14112

llvm-svn: 251907