[mir-opt] Simplify SwitchInts of explictly written ranges to range tests

[Centril]

Consider the program:

fn program(x: u8) -> u8 {
    match x {
        1 | 2 | 3 | 4 => 0,
        _ => 1,
    }
}

We will generate the following SwitchInt:

switchInt(_1) -> [1u8: bb2, 2u8: bb2, 3u8: bb2, 4u8: bb2, otherwise: bb1];

Ostensibly, something like this would be more profitable:

_2 = Le(const 1u8, _1);
switchInt(move _2) -> [false: bb2, otherwise: bb3];

This is the code you get from the range pattern 0..=4.

cc @wesleywiser @oli-obk

[oli-obk]

If there are multiple patterns it's not quite as simple.

fn program(x: u8) -> u8 {
    match x {
        1 | 2 => 2,
		3 | 4 => 0,
        _ => 1,
    }
}

will generate

switchInt(_1) -> [1u8: bb2, 2u8: bb2, 3u8: bb3, 4u8: bb3, otherwise: bb1];

which will need multiple switchInt

Or we teach switchInt to actually understand ranges, not sure that would translate to llvm in any reasonable way.