Sema: allow @ptrCast single-item pointer to slice

[mlugg]

This is a proposal, but I think it makes sense as a trivial extension of #22706. The idea there is that we know the exact number of bytes the operand slice refers to, so we can use it to compute the length of the result slice. Well, there's another case in which we know the number of bytes the operand refers to: a single-item pointer!

Notably, where #22706 allowed replacing mem.sliceAsBytes(slice) with @ptrCast(slice), this proposal allows replacing mem.asBytes(single_ptr) with @ptrCast(single_ptr) (at least, in cases where you want a slice rather than an array pointer):

test"value to byte slice with @ptrCast" {
constval: u32=1;
constbytes: []constu8=@ptrCast(&val);
switch (@import("builtin").target.cpu.arch.endian()) {
.little=>trystd.testing.expect(std.mem.eql(u8, bytes, "\x01\x00\x00\x00")),
.big=>trystd.testing.expect(std.mem.eql(u8, bytes, "\x00\x00\x00\x01")),
}
}
conststd=@import("std");