Fix optimisations with compileExprIndexExpr

[Lonegwadiwaitor]

Currently in Luau 0.607's compileExprIndexExpr:

Constant cv = getConstant(expr->index);

always returns nil, meaning the conditions that generate LOP_GETTABLEN and LOP_GETTABLEKS for string/number indexes are both unreachable, this has been fixed in this PR.

LOP_GETIMPORT can also now be generated for string indexes if optimisations are enabled.

Comparisons

Test 1 (unreachable string/number branches in compileExprIndexExpr)

Repro code:

local _ = foo["bar"]

Luau 0.607 with no optimisations
(output from luau.lonegladiator.dev, same output as luau-compile util):

PR with no optimisations:

Test 2 (LOP_GETIMPORT for compileExprIndexExpr)

Repro code:

local _ = foo["bar"]

Luau 0.607 with -O1 and -O2:

PR with -O1 and -O2:

[matthargett]

Can you add unit tests? There are examples of opcode emission tests in the codebase.

[Lonegwadiwaitor]

Modified import dumping logic in BytecodeBuilder::dumpConstant to wrap brackets around indexes with whitespace as it looked a little silly without it.

2 new test cases: IndexExprConditions & IndexExprImport

IndexExprConditions

Makes sure that GETTABLEKS is being generated for string indexes and GETTABLEN for number indexes when no optimisations are enabled.

IndexExprImport

tests GETIMPORT for compileExprIndexExpr

[vegorov-rbx]

Missing optimizations are not bugs, please fix the PR title.

For GETTABLEN and GETTABLEKS:

always returns nil

This is simply not true, constants are filled in optimization level 1.
If you need optimizations to happen, enable optimizations.
Additionally, your change breaks the constant propagation in the indexer expression.

For the getimport part, your code introduces too much code duplication and once again, breaks the computed constants on O1.
Motivation for a change is missing a bit, since you can rewrite your foo["bar"] into foo.bar, but this is still a good idea overall, so can be rewritten.

Changes are also not flagged.

The change for check if index contains whitespace is unnecessary and smells of buffer overflows since a string view like StringRef is checked with a C functions that's looking for null-termination.
It is also incomplete because there are many other cases of AstExprIndexExpr strings that are not valid in a.b.

Thank you for your interest in improving Luau, but we are not going to accept the PR in current form.