[SveEmitter] Add builtins for compares and ReverseCompare flag.

The IsReverseCompare flag tells CGBuiltin to swap the operands,
so that a LT/LE intrinsics can be expressed in terms of GE/GT
intrinsics.

This patch also adds builtins for the wide-variants of the compares.

Reviewers: SjoerdMeijer, efriedma, ctetreau

Reviewed By: efriedma

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78747