solve_bicgstab: cut use of s (#3629)

## Summary

The MF named `s` seems to be an unnecessary usage of memory as the
residue `r` can fulfill its roles in the algorithm. This PR replaces `s`
with `r` and `LinComb` with `Saxpy` as appropriate.

## Additional background

This PR is part of a larger request to improve `solve_bicgstab` and
`solve_cg`.

Src/LinearSolvers/MLMG/AMReX_MLCGSolver.H

@@ -98,7 +98,6 @@ MLCGSolverT<MF>::solve_bicgstab (MF& sol, const MF& rhs, RT eps_rel, RT eps_abs)
     MF sorig = Lp.make(amrlev, mglev, nghost);
     MF p     = Lp.make(amrlev, mglev, nghost);
     MF r     = Lp.make(amrlev, mglev, nghost);
-    MF s     = Lp.make(amrlev, mglev, nghost);
     MF rh    = Lp.make(amrlev, mglev, nghost);
     MF v     = Lp.make(amrlev, mglev, nghost);
     MF t     = Lp.make(amrlev, mglev, nghost);
@@ -166,9 +165,9 @@ MLCGSolverT<MF>::solve_bicgstab (MF& sol, const MF& rhs, RT eps_rel, RT eps_abs)
             ret = 2; break;
         }
         MF::Saxpy(sol, alpha, ph, 0, 0, ncomp, nghost); // sol += alpha * ph
-        MF::LinComb(s, RT(1.0), r, 0, -alpha, v, 0, 0, ncomp, nghost); // s = r - alpha * v
+        MF::Saxpy(r,  -alpha, v,  0, 0, ncomp, nghost); // r += -alpha * v
 
-        rnorm = norm_inf(s);
+        rnorm = norm_inf(r);
 
         if ( verbose > 2 && ParallelDescriptor::IOProcessor() )
         {
@@ -180,15 +179,15 @@ MLCGSolverT<MF>::solve_bicgstab (MF& sol, const MF& rhs, RT eps_rel, RT eps_abs)
 
         if ( rnorm < eps_rel*rnorm0 || rnorm < eps_abs ) { break; }
 
-        sh.LocalCopy(s,0,0,ncomp,nghost);
+        sh.LocalCopy(r,0,0,ncomp,nghost);
         Lp.apply(amrlev, mglev, t, sh, MLLinOpT<MF>::BCMode::Homogeneous, MLLinOpT<MF>::StateMode::Correction);
         Lp.normalize(amrlev, mglev, t);
         //
         // This is a little funky.  I want to elide one of the reductions
         // in the following two dotxy()s.  We do that by calculating the "local"
         // values and then reducing the two local values at the same time.
         //
-        RT tvals[2] = { dotxy(t,t,true), dotxy(t,s,true) };
+        RT tvals[2] = { dotxy(t,t,true), dotxy(t,r,true) };
 
         BL_PROFILE_VAR("MLCGSolver::ParallelAllReduce", blp_par);
         ParallelAllReduce::Sum(tvals,2,Lp.BottomCommunicator());
@@ -203,7 +202,7 @@ MLCGSolverT<MF>::solve_bicgstab (MF& sol, const MF& rhs, RT eps_rel, RT eps_abs)
             ret = 3; break;
         }
         MF::Saxpy(sol, omega, sh, 0, 0, ncomp, nghost); // sol += omega * sh
-        MF::LinComb(r, RT(1.0), s, 0, -omega, t, 0, 0, ncomp, nghost); // r = s - omega * t
+        MF::Saxpy(r,  -omega, t,  0, 0, ncomp, nghost); // r += -omega * t
 
         rnorm = norm_inf(r);