Change 1/2 spill slots to more general number of spill slots.

Rationale:
This prepares requesting a different number of spill slots
during SIMD vectorization.

Bug: 34083438
Test: test-art-host, test-art-host-gtest-register_allocator_test
Change-Id: I6d22966ba483deec72b5eea5061c403c12b2ada7
5 files changed