[DAGCombiner] Don't make a BUILD_VECTOR with operands of illegal type.

When DAGCombiner visits a SIGN_EXTEND_INREG of a BUILD_VECTOR with
constant operands, a new BUILD_VECTOR node will be created transformed
constants.

Llvm-stress found a case where the new BUILD_VECTOR had constant operands
of an illegal type, because the (legal) element type is in fact not a legal
scalar type.

This patch changes this so that the new BUILD_VECTOR has the same operand
type as the old one.

Review: Eli Friedman, Nirav Dave
https://bugs.llvm.org//show_bug.cgi?id=32422

llvm-svn: 299540