remove the "top of the comptime stack" compile error

It's still best practice to put `@setEvalBranchQuota` at the top of
the comptime stack, but as Jimmi notes in #1949, when a function
can be called at comptime and also can be the top of the comptime stack,
this compile error is fundamentally unsound.

So now it's gone.

closes #1949

Author: andrewrk

src/ir.cpp

@@ -18172,12 +18172,6 @@ static IrInstruction *ir_analyze_instruction_type_id(IrAnalyze *ira,
 static IrInstruction *ir_analyze_instruction_set_eval_branch_quota(IrAnalyze *ira,
         IrInstructionSetEvalBranchQuota *instruction)
 {
-    if (ira->new_irb.exec->parent_exec != nullptr && !ira->new_irb.exec->is_generic_instantiation) {
-        ir_add_error(ira, &instruction->base,
-                buf_sprintf("@setEvalBranchQuota must be called from the top of the comptime stack"));
-        return ira->codegen->invalid_instruction;
-    }
-
     uint64_t new_quota;
     if (!ir_resolve_usize(ira, instruction->new_quota->child, &new_quota))
         return ira->codegen->invalid_instruction;

test/compile_errors.zig

@@ -4554,20 +4554,6 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         ".tmp_source.zig:2:24: error: expected [2]u8 literal, found [3]u8 literal",
     );
 
-    cases.add(
-        "@setEvalBranchQuota in non-root comptime execution context",
-        \\comptime {
-        \\    foo();
-        \\}
-        \\fn foo() void {
-        \\    @setEvalBranchQuota(1001);
-        \\}
-    ,
-        ".tmp_source.zig:5:5: error: @setEvalBranchQuota must be called from the top of the comptime stack",
-        ".tmp_source.zig:2:8: note: called from here",
-        ".tmp_source.zig:1:10: note: called from here",
-    );
-
     cases.add(
         "wrong pointer implicitly casted to pointer to @OpaqueType()",
         \\const Derp = @OpaqueType();