Be less aggressive about hinting in RAFast.

In particular, don't spill dirty registers only to satisfy a hint. It is
not worth it.

The attached test case provides an example where the fast allocator
would spill a register when other registers are available.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132900 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/CodeGen/ARM/fast-isel-static.ll b/test/CodeGen/ARM/fast-isel-static.ll
index 2d79674..a86e325 100644
--- a/test/CodeGen/ARM/fast-isel-static.ll
+++ b/test/CodeGen/ARM/fast-isel-static.ll
@@ -23,7 +23,7 @@
   %z = alloca float, align 4
   store float 0.000000e+00, float* %ztot, align 4
   store float 1.000000e+00, float* %z, align 4
-; CHECK-LONG: blx     r2
+; CHECK-LONG: blx     r
 ; CHECK-NORM: bl      _myadd
   call void @myadd(float* %ztot, float* %z)
   ret i32 0