simd intrinsics with mask: accept unsigned integer masks, and fix some of the errors

[RalfJung]

It's not clear at all why the mask would have to be signed, it is anyway interpreted bitwise. The backend should just make sure that works no matter the surface-level type; our LLVM backend already does this correctly. The note of "the mask may be widened, which only has the correct behavior for signed integers" explains... nothing? Why can't the code do the widening correctly? If necessary, just cast to the signed type first...

Also while we are at it, fix the errors. For simd_masked_load/store, the errors talked about the "third argument" but they meant the first argument (the mask is the first argument there). They also used the wrong type for expected_element.

I have extremely low confidence in the GCC part of this PR.

See discussion on Zulip

[rustbot]

r? @Nadrieril

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[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred to the platform-builtins intrinsics. Make sure the
LLVM backend as well as portable-simd gets adapted for the changes.

cc @antoyo, @GuillaumeGomez, @bjorn3, @calebzulawski, @programmerjake

[RalfJung]

@bjorn3 anything we have to do here in cranelift?

[Nadrieril]

r? codegen

[rustbot]

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin