[PowerPC] Add test after #75271 on PPC. NFC. (#75616)

Demonstrate `IMPLICIT_DEF implicit-def ...` can be generated after
coalescing on PPC.

The case is reduced from failure in #75570. The failure is triggered
after #75271 .

Author: Kai Luo

llvm/test/CodeGen/PowerPC/coalescer-remat-with-undef-implicit-def-operand.mir

@@ -0,0 +1,28 @@
+# RUN: llc -mtriple=powerpc64le-unknown-linux-gnu -verify-coalescing -run-pass=register-coalescer \
+# RUN:   -o - %s | FileCheck %s
+---
+name:            _Z13testTransposeIfLj31ELj17EEvv
+alignment:       16
+tracksRegLiveness: true
+frameInfo:
+  maxAlignment:    128
+machineFunctionInfo: {}
+body:             |
+  ; CHECK-LABEL: name:            _Z13testTransposeIfLj31ELj17EEvv
+  ; CHECK: undef %[[REG:[0-9]+]].sub_64:vsrc = IMPLICIT_DEF implicit-def %[[REG]]
+  bb.0:
+    liveins: $x2
+    %2:vssrc = IMPLICIT_DEF
+    B %bb.2
+
+  bb.1:
+    %0:vsrc = SUBREG_TO_REG 1, killed %2, %subreg.sub_64
+    %1:vsrc = XXPERMDI killed undef %0, killed %0, 0
+    BLR8 implicit $lr8, implicit $rm
+
+  bb.2:
+    successors: %bb.2(0x7c000000), %bb.1(0x04000000)
+    BDNZ8 %bb.2, implicit-def $ctr8, implicit $ctr8
+    B %bb.1
+
+...