Revert "[DAG] Extend visitSCALAR_TO_VECTOR optimization to truncated vector."

This reverts commit r309680 which appears to be raising an assertion
in the test-suite.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309717 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed