[X86][test] Pre-commit tests for #95412

Author: KanRobert

llvm/test/MC/Disassembler/X86/apx/rex2-format.txt

@@ -21,6 +21,12 @@
 # INTEL: movsxd	r17, r16d
 0xd5,0x58,0x63,0xc8
 
+# ATT: rep
+# ATT-SAME:   popcntl %r16d, %r17d
+# INTEL: rep
+# INTEL-SAME: popcnt r17d, r16d
+0xf3,0xd5,0xd0,0xb8,0xc8
+
 ## MRMSrcRegCC
 
 # ATT:   cmovll	%r16d, %eax

llvm/test/MC/X86/apx/rex2-format-att.s

@@ -20,6 +20,10 @@
 # CHECK: encoding: [0xd5,0x58,0x63,0xc8]
          movslq	%r16d, %r17
 
+# CHECK: popcntl %r16d, %r17d
+# CHECK: encoding: [0xf3,0xd5,0xd0,0xb8,0xc8]
+         popcntl %r16d, %r17d
+
 ## MRMSrcRegCC
 
 # CHECK: cmovll	%r16d, %eax

llvm/test/MC/X86/apx/rex2-format-intel.s

@@ -20,6 +20,10 @@
 # CHECK: encoding: [0xd5,0x58,0x63,0xc8]
          movsxd	r17, r16d
 
+# CHECK: popcnt r17d, r16d
+# CHECK: encoding: [0xf3,0xd5,0xd0,0xb8,0xc8]
+         popcnt r17d, r16d
+
 ## MRMSrcRegCC
 
 # CHECK: cmovl	eax, r16d