Fix a recent regression in Applications/sgefa that Alkis pointed out to me.

Fix a recent regression in Applications/sgefa that Alkis pointed out to me.
The vector may actually be empty if the register that we are marking as
recently used is not actually allocatable.  This happens for physical registers
that are not allocatable, like the ST(x) registers on X86.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@14195 91177308-0d34-0410-b5e6-96231b3b80d8