Sat shl neg rhs

[travisstaloch]

This pr adds a check for negative rhs to saturating shl and expected compile errors test cases.

This is a follow up to my saturating arithmetic pr and was discovered by @Snektron as demonstrated in the following:

pub fn main() void {
    _ = @as(i32, 1) <<| @as(i32, -2);
}

This pr prevents the compiler bug and emits a compile error:

$ ./zig run tmp.zig 
./tmp.zig:6:25: error: shift by negative value -2
    _ = @as(i32, 1) <<| @as(i32, -2);
                        ^
./tmp.zig:6:21: note: referenced here
    _ = @as(i32, 1) <<| @as(i32, -2);
                    ^

[travisstaloch]

I forgot to mention that the compiler bug without this patch is:

$ ../build/zig run tmp.zig
Assertion failed at ../src/stage1/bigint.cpp:1432 in bigint_shl. This is a bug in the Zig compiler.thread 26254 panic: 
/media/data/Users/Travis/Documents/Code/zig/zig/src/stage1.zig:177:5: 0x559a5da669a7 in stage2_panic (zig1)
    @panic(ptr[0..len]);
    ^
../src/stage1/util.cpp:20:17: 0x559a5e9222d2 in zig_panic (../src/stage1/util.cpp)
    stage2_panic("", 0);
                ^
../src/stage1/util_base.hpp:53:18: 0x559a5ea5aaaa in zig_assert (../src/stage1/bigint.cpp)
        zig_panic("Assertion failed at %s:%d in %s. This is a bug in the Zig compiler.", file, line, func);
                 ^
../src/stage1/bigint.cpp:1432:5: 0x559a5ea5f2f1 in bigint_shl (../src/stage1/bigint.cpp)
    assert(!op2->is_negative);
    ^
../src/stage1/bigint.cpp:545:15: 0x559a5ea5c51b in bigint_shl_sat (../src/stage1/bigint.cpp)
    bigint_shl(dest, op1, op2);
              ^
../src/stage1/ir.cpp:9846:31: 0x559a5e99400f in ir_eval_math_op_scalar (../src/stage1/ir.cpp)
                bigint_shl_sat(&out_val->data.x_bigint, &op1_val->data.x_bigint, &op2_val->data.x_bigint, type_entry->data.integral.bit_count, type_entry->data.integral.is_signed);
                              ^
../src/stage1/ir.cpp:9896:35: 0x559a5e994331 in ir_analyze_math_op (../src/stage1/ir.cpp)
        if (ir_eval_math_op_scalar(ira, scope, source_node, type_entry, op1_val, op_id, op2_val, out_val) != nullptr) {
                                  ^
../src/stage1/ir.cpp:10412:34: 0x559a5e995f00 in ir_analyze_bin_op_math (../src/stage1/ir.cpp)
        return ir_analyze_math_op(ira, instruction->base.scope, instruction->base.source_node, resolved_type, op1_val, op_id, op2_val);
                                 ^
../src/stage1/ir.cpp:11053:42: 0x559a5e998895 in ir_analyze_instruction_bin_op (../src/stage1/ir.cpp)
            return ir_analyze_bin_op_math(ira, bin_op_instruction);
                                         ^
...