Handle SHL with a constant vector in LLVMVectorIsLinear().

LLVM3.4 seems to be turning multiplies by a constant power of 2 into
the equivalent SHL, which was in turn thwarting the pattern matching
for turning gathers/scatters into vector loads/stores.
Merge pull request #544 from mmp/master

Handle SHL with a constant vector in LLVMVectorIsLinear().