Fix vector splitting for extract_vector_elt and vector elements of <8-bits.

Summary:
One of the vector splitting paths for extract_vector_elt tries to lower:
    define i1 @via_stack_bug(i8 signext %idx) {
      %1 = extractelement <2 x i1> <i1 false, i1 true>, i8 %idx
      ret i1 %1
    }
to:
    define i1 @via_stack_bug(i8 signext %idx) {
      %base = alloca <2 x i1>
      store <2 x i1> <i1 false, i1 true>, <2 x i1>* %base
      %2 = getelementptr <2 x i1>, <2 x i1>* %base, i32 %idx
      %3 = load i1, i1* %2
      ret i1 %3
    }
However, the elements of <2 x i1> are not byte-addressible. The result of this
is that the getelementptr expands to '%base + %idx * (1 / 8)' which simplifies
to '%base + %idx * 0', and then simply '%base' causing all values of %idx to
extract element zero.

This commit fixes this by promoting the vector elements of <8-bits to i8 before
splitting the vector.

This fixes a number of test failures in pocl.

Reviewers: pekka.jaaskelainen

Subscribers: pekka.jaaskelainen, llvm-commits

Differential Revision: http://reviews.llvm.org/D12591

llvm-svn: 247128