See Note [Inlining in ArgCtxt].  This very small change gives quite a
big performance win. Just showing the bigger ones:

        Program           Size    Allocs   Runtime
--------------------------------------------------------------------------------
           anna          -0.7%     -4.3%      0.15
       cichelli          -0.6%     -6.4%      0.15
         fulsom          -0.4%    -18.5%     -8.1%
            gcd          -0.6%    -12.0%      0.06
        integer          -0.6%    -16.2%     -8.4%
          power          -0.7%    -19.3%     -4.8%
--------------------------------------------------------------------------------
            Min          -0.7%    -19.3%    -15.7%
            Max          -0.1%     +0.1%     +5.7%
 Geometric Mean          -0.6%     -1.9%     -4.3%

The original change was to improve a case that Roman found (see test
eyeball/inline1) but that seems to work ok now anyway.