Revert commit 0e46b49de433 from llvm-project (by Matt Arsenault):
Reapply "RegisterCoalescer: Add implicit-def of super register when coalescing SUBREG_TO_REG" This reverts commit c398fa009a47eb24f88383d5e911e59e70f8db86. PPC backend was fixed in 2f82662ce901c6666fceb9c6c5e0de216a1c9667
Since it causes an assertion failure building /sys/dev/fb/vga.c:
https://github.com/llvm/llvm-project/issues/76416
PR: 276104
MFC after: 1 month
(cherry picked from commit edc2dc17b1f2dfe45dc85e6cc0ff54bca1ac8214)