RegAllocFast: Remove early selection loop, the spill calculation will report...

RegAllocFast: Remove early selection loop, the spill calculation will report cost 0 anyway for free regs

The 2nd loop calculates spill costs but reports free registers as cost
0 anyway, so there is little benefit from having a separate early
loop.

Surprisingly this is not NFC, as many register are marked regDisabled
so the first loop often picks up later registers unnecessarily instead
of the first one available in the allocation order...

Patch by Matthias Braun

llvm-svn: 356499