Modernize InvertPSDMatrix with if constexpr

Use if constexpr for kSize checks in InvertPSDMatrix to allow for
dead-code elimination at compile time. This ensures that for small
fixed-size matrices, only the inverse() path is compiled, and for
larger or dynamic matrices, only the LLT path is compiled when
assume_full_rank is true.

Change-Id: I4def2faadd080a4defccf1c0527015ae004f20ce

internal/ceres/invert_psd_matrix.h

diff --git a/internal/ceres/invert_psd_matrix.h b/internal/ceres/invert_psd_matrix.h
index 50b357c..78ee45b 100644
--- a/internal/ceres/invert_psd_matrix.h
+++ b/internal/ceres/invert_psd_matrix.h
@@ -60,11 +60,12 @@
   //
   // https://eigen.tuxfamily.org/dox/group__TutorialLinearAlgebra.html#title3
   if (assume_full_rank) {
-    if (kSize > 0 && kSize < 5) {
+    if constexpr (kSize > 0 && kSize < 5) {
       return m.inverse();
+    } else {
+      return m.template selfadjointView<Eigen::Upper>().llt().solve(
+          MType::Identity(size, size));
     }
-    return m.template selfadjointView<Eigen::Upper>().llt().solve(
-        MType::Identity(size, size));
   }
 
   // For a thin SVD the number of columns of the matrix need to be dynamic.