Proposal: apply RLS to @splat, eliminating its length parameter

[mlugg]

Status quo:

const x = @splat(4, @as(u32, 123)); // .{ 123, 123, 123, 123 }

Proposal:

// result type used to infer length and elem type
const x: @Vector(4, u32) = @splat(123); // .{ 123, 123, 123, 123 }

This brings similar advantages to #5909, and fits nicely with it. @splat's current usage is quite unpleasant for constants in particular, due to the need to annotate the element type with @as - this would reduce visual noise in most cases.