Add a separate list of fixed intervals. This improves the running time

Add a separate list of fixed intervals. This improves the running time
of the register allocator as follows:

       before   after
mesa   2.3790  1.5994
vpr    2.6008  1.2078
gcc    1.9840  0.5273
mcf    0.2569  0.0470
eon    1.8468  1.4359
twolf  0.9475  0.2004
burg   1.6807  1.3300
lambda 1.2191  0.3764

Speedups range anyware from 30% to over 400% :-)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10712 91177308-0d34-0410-b5e6-96231b3b80d8