Correctly terminate a physreg redefined by an early clobber.

I don't have a test that fails because of this, but a test case like
CodeGen/X86/2009-12-01-EarlyClobberBug.ll exposes the problem.  EAX is
redefined by a tied early clobber operand on inline asm, and the live
range should look like this:

  %EAX,inf = [48r,64e:0)[64e,80r:1)  0@48r 1@64e

Previously, the two values got merged:

  %EAX,inf = [48r,80r:0)  0@48r

With this bug fixed, the REDEF_BY_EC VNInfo flag is no longer needed.

llvm-svn: 149768