[lang] MatrixType bug fix: Fix array indexing with MatrixType-index

[jim19930609]

Issue: #5819

Brief Summary

Taichi supports indexing field/ndarray/... using MatrixTyped index. For example:

x = ti.field(shape=(2, 4), ti.i32)

@ti.kernel
def test():
  base = ti.Vector([1, 1])
  x[base] = 100

After this PR, the above test case works under real_matrix and real_matrix_scalarize

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[strongoier]

LGTM!