Sema: remove incorrect safety check for saturating left shift

[Justus2308]

Closes #23033

This seems to do it.

pub const panic = @import("std").debug.no_panic;
var lhs: u3 = 1;
var rhs: u1 = 0;
export fn shlSat() void {
    lhs <<|= rhs;
}

zig build-obj repro.zig --verbose-air

old

thread 172206856 panic: reached unreachable code
# Begin Function AIR: repro.shlSat:
# Total AIR+Liveness bytes: 469B
# AIR Instructions:         21 (189B)
# AIR Extra Data:           28 (112B)
# Liveness tomb_bits:       16B
# Liveness Extra Data:      6 (24B)
# Liveness special table:   2 (16B)
  %13 = alloc(*[32]usize)
  %14 = alloc(*builtin.StackTrace)
  %15 = struct_field_ptr_index_1(*[]usize, %14)
  %16 = array_to_slice([]usize, %13!)
  %17!= store(%15!, %16!)
  %18 = struct_field_ptr_index_0(*usize, %14)
  %19!= store(%18!, @.zero_usize)
  %20!= set_err_return_trace(%14!)
  %0!= save_err_return_trace_index()
  %1!= dbg_stmt(2:5)
  %2 = load(u3, <*u3, &repro.lhs>)
  %3 = load(u1, <*u1, &repro.rhs>)
  %4 = cmp_lt(%3, <u1, 3>)
  %7!= block(void, {
    %8!= cond_br(%4!, likely {
      %9!= br(%7, @.void_value)
    }, cold {
      %2! %3!
      %5!= call(<fn () noreturn, (function 'shiftRhsTooBig')>, [])
      %6!= unreach()
    })
  } %4!)
  %10 = shl_sat(%2!, %3!)
  %11!= store_safe(<*u3, &repro.lhs>, %10!)
  %12!= ret_safe(@.void_value)
# End Function AIR: repro.shlSat

[stack trace...]

new

# Begin Function AIR: repro.shlSat:
# Total AIR+Liveness bytes: 219B
# AIR Instructions:         7 (63B)
# AIR Extra Data:           9 (36B)
# Liveness tomb_bits:       8B
# Liveness Extra Data:      0 (0B)
# Liveness special table:   0 (0B)
  %0!= save_err_return_trace_index()
  %1!= dbg_stmt(2:5)
  %2 = load(u3, <*u3, &repro.lhs>)
  %3 = load(u1, <*u1, &repro.rhs>)
  %4 = shl_sat(%2!, %3!)
  %5!= store_safe(<*u3, &repro.lhs>, %4!)
  %6!= ret_safe(@.void_value)
# End Function AIR: repro.shlSat