[SelectionDAG] Enhance SDTCisSameNumEltsAs to work with scalar types and use...

[SelectionDAG] Enhance SDTCisSameNumEltsAs to work with scalar types and use it on extend/trunc/round operations.

Currently we don't enforce that ISD::ANY_EXTEND, ZERO_EXTEND, SIGN_EXTEND, TRUNC, FP_ROUND, FP_EXTEND have the same number of elements(including scalar) between their input and output. Though we have them documented as such. Up until a few months ago x86 created nodes that violated this rule. That's all been fixed now, and we should enforce the rule going forward.

In order to do this we need to allow SDTCisSameNumEltsAs to support scalar types and not enforce being a vector. If one type is scalar we will force the other type to also be scalar.

Differential Revision: https://reviews.llvm.org/D30878

llvm-svn: 297648