A compiler may not implicitly cast the output of first->spill_type() to an integer, adding a cast to fix the problem

Change-Id: Ic4e779d447fba9d040b81ee315327b631dfd6ad3
Reviewed-on: https://chromium-review.googlesource.com/c/1480913Reviewed-by: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59786}