ARM64: handle v1i1 types arising from setcc properly.

There were several overlapping problems here, and this solution is
closely inspired by the one adopted in AArch64 in r201381.

Firstly, scalarisation of v1i1 setcc operations simply fails if the
input types are legal. This is fixed in LegalizeVectorTypes.cpp this
time, and allows AArch64 code to be simplified slightly.

Second, vselect with such a setcc feeding into it ends up in
ScalarizeVectorOperand, where it's not handled. I experimented with an
implementation, but found that whatever DAG came out was rather
horrific. I think Hao's DAG combine approach is a good one for
quality, though there are edge cases it won't catch (to be fixed
separately).

Should fix PR19335.
DAGLegalize: add last-ditch type-legalization for VSELECT.

When LLVM sees something like (v1iN (vselect v1i1, v1iN, v1iN)) it can
decide that the result is OK (v1i64 is legal on AArch64, for example)
but it still need scalarising because of that v1i1. There was no code
to do this though.

AArch64 and ARM64 have DAG combines to produce efficient code and
prevent that occuring in *most* such situations, but there are edge
cases that they miss. This adds a legalization to cope with that.