Fix a long-standing spiller bug:

Fix a long-standing spiller bug:
If a spillslot value is available in a register, and there is a noop copy that
targets that register, the spiller correctly decide not to invalidate the
spillslot register.

However, even though the noop copy does not clobbers the value. It does start a
new intersecting live range. That means the spillslot register is available for
use but should not be reused for a two-address instruction modref operand which
would clobber the new live range.

When we remove the noop copy, update the available information by clearing the
canClobber bit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@32576 91177308-0d34-0410-b5e6-96231b3b80d8