[Prism] Fix IfNode and ElseNode

ElseNode looks to have been implemented at the same time as IfNode, but
was resulting in a stack underflow error.

The following is from the test code

```
if foo
  bar
end
```

```
❯ make run
compiling compile.c
linking miniruby
./miniruby -I./lib -I. -I.ext/common  -r./arm64-darwin22-fake  ./test.rb
CRUBY: **************************************************
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(2,19)>
0000 putself                                                          (   2)[Li]
0001 opt_send_without_block                 <calldata!mid:foo, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0003 branchunless                           9
0005 putself
0006 opt_send_without_block                 <calldata!mid:bar, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0008 pop
0009 putobject_INT2FIX_1_
0010 leave
PRISM: **************************************************
-- raw disasm--------
   0000 putself                                                          (   2)
   0001 send                 <calldata:foo, 0>, nil                      (   2)
   0004 branchunless         <L001>                                      (   2)
   0006 jump                 <L000>                                      (   2)
 <L000> [sp: 0]
*  0008 pop                                                              (   1)
   0009 putself                                                          (   2)
   0010 send                 <calldata:bar, 0>, nil                      (   2)
   0013 pop                                                              (   2)
   0014 jump                 <L002>                                      (   1)
 <L001> [sp: 0]
 <L002> [sp: -1]
   0016 putobject            1                                           (   2)
   0018 leave                                                            (   1)
---------------------
<compiled>: <compiled>:1: argument stack underflow (-1) (SyntaxError)
make: *** [run] Error 1
```

This commit fixes the stack underflow error for both IfNode and ElseNode
and introduces tests for them.

1 files changed, 12 insertions, 0 deletions

diff --git a/test/ruby/test_compile_prism.rb b/test/ruby/test_compile_prism.rb
index 9c2f232396..f0e389d30a 100644
--- a/test/ruby/test_compile_prism.rb
+++ b/test/ruby/test_compile_prism.rb
@@ -338,6 +338,18 @@ module Prism
       test_prism_eval("false || 1")
     end
 
+    def test_IfNode
+      test_prism_eval("if true; 1; end")
+      test_prism_eval("1 if true")
+    end
+
+    def test_ElseNode
+      test_prism_eval("if false; 0; else; 1; end")
+      test_prism_eval("if true; 0; else; 1; end")
+      test_prism_eval("true ? 1 : 0")
+      test_prism_eval("false ? 0 : 1")
+    end
+
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