[NFC]Fix 2 logic dead code

[NFC]Fix 2 logic dead code

First, in CodeGenPrepare.cpp, line 6891, the VectorCond will always be false
because if not function will return at 6888.
Second, in SelectionDAGBuilder.cpp, line 5443, getSExtValue() will return
value as int type, but now we use unsigned Val to maintain it, which make the
if condition at 5452 meaningless.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D149033