[lang] MatrixType bug fix: Allow indexing a matrix r-value

[strongoier]

Issue: #5819

Brief Summary

It is possible that a local matrix is indexed without being assigned to a variable:

@ti.kernel
def local_vector_read(i: ti.i32) -> ti.i32:
    return ti.Vector([0, 1, 2])[i]

In this case we need to turn it from an r-value to an l-value so that we can successfully construct the index.

[netlify]

✅ Deploy Preview for docsite-preview canceled.

Name	Link
🔨 Latest commit	a1374dc
🔍 Latest deploy log	https://app.netlify.com/sites/docsite-preview/deploys/634526fc7be7e0000af3b86a