[SveEmitter] Explicitly merge with zero/undef

Builtins that have the merge type MergeAnyExp or MergeZeroExp,
merge into a 'undef' or 'zero' vector respectively, which enables the
_x and _z behaviour for unary operations.

This patch also adds builtins for svabs and svneg.

Reviewers: SjoerdMeijer, efriedma, rovka

Reviewed By: efriedma

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77591