[SVE][CodeGen] Relax all true isel requirement for predicated operations that have no side effects.

We have isel patterns to allow predicated operations to emit
unpredicated instructions when the predicate is all true. However,
the predicated operations named #_PRED have no requirement for the
result of the inactive lanes and so when those operations have no
side effects, floating point exceptions for example, we can also
safely emit unpredicated instructions. Doing this allows better
register allocation, instruction scheduling and also enables more
usage of instructions that take immediate operands.

NOTE: This patch does not convert all possible instances but
instead focuses on the cases that are testable once D141937 lands.

Depends on D141937

Differential Revision: https://reviews.llvm.org/D141938