[turbofan] Try to allocate preferred register after splitting range.

This shows an improvement in the code size of the bytecode handlers.
When a range is split (because for example the preferred register gets
clobbered by a call and is not available for the whole range), trying
to allocate the preferred register for the first range that results
from the split avoids some extra register moves.

BUG=

Review-Url: https://codereview.chromium.org/2749023005
Cr-Commit-Position: refs/heads/master@{#43905}

src/compiler/register-allocator.cc

@@ -3152,6 +3152,9 @@ bool LinearScanAllocator::TryAllocateFreeReg(
     // the range end. Split current at position where it becomes blocked.
     LiveRange* tail = SplitRangeAt(current, pos);
     AddToUnhandledSorted(tail);
+
+    // Try to allocate preferred register once more.
+    if (TryAllocatePreferredReg(current, free_until_pos)) return true;
   }
 
   // Register reg is available at the range start and is free until the range