[llvm] [ARM] Switch to soft promoting half types. (PR #80440)

[Harald van Dijk via llvm-commits]

================
@@ -2,78 +2,113 @@
 
 define arm_aapcs_vfpcc { <8 x half>, <8 x half> } @f1() {
 ; CHECK-LABEL: _f1
-; CHECK: vpush   {d8}
-; CHECK-NEXT: vmov.f64        d8, #5.000000e-01
-; CHECK-NEXT: vmov.i32        d8, #0x0
-; CHECK-NEXT: vmov.i32        d0, #0x0
-; CHECK-NEXT: vmov.i32        d1, #0x0
-; CHECK-NEXT: vmov.i32        d2, #0x0
-; CHECK-NEXT: vmov.i32        d3, #0x0
-; CHECK-NEXT: vmov.i32        d4, #0x0
-; CHECK-NEXT: vmov.i32        d5, #0x0
-; CHECK-NEXT: vmov.i32        d6, #0x0
-; CHECK-NEXT: vmov.i32        d7, #0x0
-; CHECK-NEXT: vmov.f32        s1, s16
-; CHECK-NEXT: vmov.f32        s3, s16
-; CHECK-NEXT: vmov.f32        s5, s16
-; CHECK-NEXT: vmov.f32        s7, s16
-; CHECK-NEXT: vmov.f32        s9, s16
-; CHECK-NEXT: vmov.f32        s11, s16
-; CHECK-NEXT: vmov.f32        s13, s16
-; CHECK-NEXT: vmov.f32        s15, s16
-; CHECK-NEXT: vpop    {d8}
+; CHECK:      vpush   {d8, d9, d10, d11}
+; CHECK-NEXT: vmov.i32        q8, #0x0
+; CHECK-NEXT: vmov.u16        r0, d16[0]
+; CHECK-NEXT: vmov    d4, r0, r0
+; CHECK-NEXT: vmov.u16        r0, d16[1]
+; CHECK-NEXT: vmov    d8, r0, r0
+; CHECK-NEXT: vmov.u16        r0, d16[2]
+; CHECK-NEXT: vmov    d5, r0, r0
+; CHECK-NEXT: vmov.u16        r0, d16[3]
+; CHECK-NEXT: vmov    d9, r0, r0
+; CHECK-NEXT: vmov.u16        r0, d17[0]
+; CHECK-NEXT: vmov    d6, r0, r0
+; CHECK-NEXT: vmov.u16        r0, d17[1]
+; CHECK-NEXT: vmov    d10, r0, r0
+; CHECK-NEXT: vmov.u16        r0, d17[2]
+; CHECK-NEXT: vmov    d7, r0, r0
+; CHECK-NEXT: vmov.u16        r0, d17[3]
+; CHECK-NEXT: vmov    d11, r0, r0
+; CHECK:      vmov.f32        s0, s8
+; CHECK:      vmov.f32        s1, s16
+; CHECK:      vmov.f32        s2, s10
+; CHECK:      vmov.f32        s3, s18
+; CHECK:      vmov.f32        s4, s12
+; CHECK:      vmov.f32        s5, s20
+; CHECK:      vmov.f32        s6, s14
+; CHECK:      vmov.f32        s7, s22
+; CHECK:      vmov.f32        s9, s16
+; CHECK:      vmov.f32        s11, s18
+; CHECK:      vmov.f32        s13, s20
+; CHECK:      vmov.f32        s15, s22
+; CHECK:      vpop    {d8, d9, d10, d11}
----------------
hvdijk wrote:

We end up during ISel with
```
Creating fp constant: t2: v8f16 = BUILD_VECTOR ConstantFP:f16<APFloat(0)>, ConstantFP:f16<APFloat(0)>, ConstantFP:f16<APFloat(0)>, ConstantFP:f16<APFloat(0)>, ConstantFP:f16<APFloat(0)>, ConstantFP:f16<APFloat(0)>, ConstantFP:f16<APFloat(0)>, ConstantFP:f16<APFloat(0)>
Creating new node: t3: v8f16,v8f16 = merge_values t2, t2
Creating new node: t5: f16 = extract_vector_elt t3, Constant:i32<0>
```
and quite a lot more of those, where we should be able to see that we're extracting a constant element of a constant vector, but don't appear to have any logic anywhere yet for handling that. Or if we do, where it's not working.

https://github.com/llvm/llvm-project/pull/80440