%reg1025 = <sext> %reg1024
    ...
   %reg1026 = SUBREG_TO_REG 0, %reg1024, 4

into this:

   %reg1025 = <sext> %reg1024
    ...
   %reg1027 = EXTRACT_SUBREG %reg1025, 4
   %reg1026 = SUBREG_TO_REG 0, %reg1027, 4

The problem here is that SUBREG_TO_REG is there to assert that an implicit zext
occurs. It doesn't insert a zext instruction. If we allow the EXTRACT_SUBREG
here, it will give us the value after the <sext>, not the original value of
%reg1024 before <sext>.

llvm-svn: 105741