[ARM,MVE] Add intrinsics for FP rounding operations.

Summary:
This adds the unpredicated forms of six different MVE intrinsics which
all round a vector of floating-point numbers to integer values,
leaving them still in FP format, differing only in rounding mode and
exception settings.

Five of them map to existing target-independent intrinsics in LLVM IR,
such as @llvm.trunc and @llvm.rint. The sixth, mapping to the `vrintn`
instruction, is done by inventing a target-specific intrinsic.

(`vrintn` behaves the same as `vrintx` in terms of the output value:
the side effects on the FPSCR flags are the only difference between
the two. But ACLE specifies separate user-callable intrinsics for the
two, so the side effects matter enough to make sure we generate the
right one of the two instructions in each case.)

Reviewers: dmgreen, miyuki, MarkMurrayARM, ostannard

Reviewed By: miyuki

Subscribers: kristof.beyls, hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D74333

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