Implement dead code elimination for constant conditionals

[fglock]

Summary

This PR implements dead code elimination for constant conditionals, enabling CPAN modules like IPC::System::Simple and File::ShareDir to work correctly.

Problem

CPAN modules often use patterns like:

use constant WINDOWS => 0;
if (WINDOWS) {
    my $x = UNDEFINED_BAREWORD;  # Windows-specific code
}

Native Perl optimizes away the dead if branch at compile time, so undefined barewords in dead code don't cause errors. PerlOnJava was compiling both branches, causing "Bareword not allowed" errors.

Solution

1. constant.pm: Updated to store constants directly in the stash as references ($stash->{$name} = \$value) like native Perl's constant.pm, instead of creating subroutines. This ensures RuntimeStashEntry sets constantValue on the RuntimeCode.

2. EmitStatement.emitIf(): Added getConstantConditionValue() method to detect compile-time constant conditions (literal numbers, strings, or constant subroutine calls) and skip dead branches entirely.

3. BytecodeCompiler.visit(IfNode): Same dead code elimination for the bytecode interpreter backend.

4. POSIX.pm: Export additional constants (O_RDWR, O_CREAT, WNOHANG, WUNTRACED) needed by File::ShareDir.

5. Internals::SvREADONLY: Fixed corruption of array/hash references when called with SvREADONLY($arrayref, 1). Now properly handles ARRAYREFERENCE, HASHREFERENCE, REFERENCE, CODE, and GLOBREFERENCE types by leaving them unchanged.

Test Results

• All 163 unit tests pass (100% pass rate)
• Works for if, unless, elsif
• Works for numeric constants (0, 1), string constants ("", "yes"), and hash-style use constant
• File::ShareDir: All 54 subtests now pass (was failing on POSIX constants)
• use constant ARR => [1,2,3] now works correctly

Test plan

• make passes
• Unit tests pass
• Manual testing with IPC::System::Simple-like patterns
• File::ShareDir tests pass

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