[SVE] Replace remaining _MERGE_OP1 nodes with _PRED variants.

[SVE] Replace remaining _MERGE_OP1 nodes with _PRED variants.

This is the final bit of work to relax the register allocation
requirements when code generating normal LLVM IR, which rarely
care about the result of inactive lanes. By using _PRED nodes
we can make better use of SVE's reversed instructions.

Also removes a redundant parameter from the min/max tests.

Differential Revision: https://reviews.llvm.org/D85142