Fix *_EXTEND_VECTOR_INREG legalization

Summary:
While promoting *_EXTEND_VECTOR_INREG nodes whose inputs are already
promoted, perform the appropriate sign extension for the promoted node
before doing the *_EXTEND_VECTOR_INREG operation.  If not, the undefined
high-order bits of the promoted operand may (a) be garbage inc ase of
zext) or (b) contribute the wrong sign-bit (in case of sext)

Updated the promote-vec3.ll test after this change.  The diff shows
explicit zeroing in case of zext and intermediate sign extension in case
of sext.

Reviewers: RKSimon

Subscribers: llvm-commits, srhines

Differential Revision: https://reviews.llvm.org/D25790

llvm-svn: 284752