[llvm] [AMDGPU][GlobalISel] Add RegBankLegalize support for G_AMDGPU_MAD_* (PR #176060)

[Petar Avramovic via llvm-commits]

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@@ -716,6 +717,131 @@ bool RegBankLegalizeHelper::lowerSplitTo16(MachineInstr &MI) {
   return true;
 }
 
+bool RegBankLegalizeHelper::lowerUniMAD64(MachineInstr &MI) {
+  Register Dst0 = MI.getOperand(0).getReg();
+  Register Dst1 = MI.getOperand(1).getReg();
+  Register Src0 = MI.getOperand(2).getReg();
+  Register Src1 = MI.getOperand(3).getReg();
+  Register Src2 = MI.getOperand(4).getReg();
+
+  bool IsUnsigned = MI.getOpcode() == AMDGPU::G_AMDGPU_MAD_U64_U32;
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petar-avramovic wrote:

this one
```
define amdgpu_ps void @s_mul_u64_sext_with_sregs(ptr addrspace(1) %out, i32 %val) {
  %ext = sext i32 %val to i64
  %mul = mul i64 %ext, 80
  store i64 %mul, ptr addrspace(1) %out, align 8
  ret void
}
```
will result with signed v_mad_co_i64_i32 on gfx12+
for other missing cases it is probably best to deal with them after regbanklegalize and fold only for vgpr inputs

https://github.com/llvm/llvm-project/pull/176060