[ARM] Do not scale vext with a factor

The vext pseudo-instruction takes the number of elements that need to be
extracted, not the number of bytes. Hence, use the number of elements
directly instead of scaling them with a factor.

Reviewers: Silviu Baranga, James Molloy
(not reflected in the differential revision)

Differential Revision: http://reviews.llvm.org/D12974

llvm-svn: 248208