Legalizer: Use the scalar bit width when promoting bit counting instrs on

vectors.

e.g. when promoting ctlz from <2 x i32> to <2 x i64> we have to fixup
the result by 32 bits, not 64. PR20917.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217671 91177308-0d34-0410-b5e6-96231b3b80d8