Work around CCE 19.0.0 optcg ICE in m_phase_change.fpp

Replace matmul() with explicit 2x2 matrix-vector multiply to avoid
an intermittent CCE 19.0.0 internal compiler error (signal 13 segfault
in optcg) when matmul is used inside a subroutine marked with both
!$acc routine seq and !DIR$ INLINEALWAYS.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Spencer Bryngelson

src/common/m_phase_change.fpp

@@ -579,7 +579,8 @@ contains
             InvJac = InvJac/(Jac(1, 1)*Jac(2, 2) - Jac(1, 2)*Jac(2, 1))
 
             ! calculating correction array for Newton's method
-            DeltamP = -1.0_wp*(matmul(InvJac, R2D))
+            DeltamP(1) = -1.0_wp*(InvJac(1, 1)*R2D(1) + InvJac(1, 2)*R2D(2))
+            DeltamP(2) = -1.0_wp*(InvJac(2, 1)*R2D(1) + InvJac(2, 2)*R2D(2))
 
             ! updating two reacting 'masses'. Recall that inert 'masses' do not change during the phase change
             ! liquid