[AArch64][GlobalISel] Widen G_SELECT before clamping it

[paquette]

Summary

This allows us to handle the s88 G_SELECTS:

https://godbolt.org/z/5s18M4erY

Weird types like this can result in weird merges.

Widening to s128 first and then clamping down avoids that situation.

[jroelofs]

LGTM

llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp

getActionDefinitionsBuilder({G_SITOFP, G_UITOFP})
       .widenScalarToNextPow2(0);
 
   // Control-flow
   getActionDefinitionsBuilder(G_BRCOND).legalFor({s1, s8, s16, s32});
   getActionDefinitionsBuilder(G_BRINDIRECT).legalFor({p0});
 
   getActionDefinitionsBuilder(G_SELECT)
       .legalFor({{s32, s1}, {s64, s1}, {p0, s1}})
-      .clampScalar(0, s32, s64)
       .widenScalarToNextPow2(0)
+      .clampScalar(0, s32, s64)
       .minScalarEltSameAsIf(all(isVector(0), isVector(1)), 1, 0)
       .lowerIf(isVector(0));
 
   // Pointer-handling
   getActionDefinitionsBuilder(G_FRAME_INDEX).legalFor({p0});
 
   if (TM.getCodeModel() == CodeModel::Small)
     getActionDefinitionsBuilder(G_GLOBAL_VALUE).custom();

llvm/test/CodeGen/AArch64/GlobalISel/legalize-select.mir

bb.1:
     %6:_(s32) = G_FCONSTANT float 0.000000e+00
     %5:_(<4 x s32>) = G_BUILD_VECTOR %6(s32), %6(s32), %6(s32), %6(s32)
     %3:_(s1) = G_ICMP intpred(eq), %0(s32), %2
     %4:_(<4 x s32>) = G_SELECT %3(s1), %1, %5
     $q0 = COPY %4(<4 x s32>)
     RET_ReallyLR implicit $q0
 
 ...
+---
+name:            s88
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $w1, $x0
+
+    ; CHECK-LABEL: name: s88
+    ; CHECK: liveins: $w0, $w1, $x0
+    ; CHECK: %a:_(s32) = COPY $w0
+    ; CHECK: %b:_(s32) = COPY $w1
+    ; CHECK: [[ICMP:%[0-9]+]]:_(s32) = G_ICMP intpred(sgt), %a(s32), %b
+    ; CHECK: %cmp:_(s1) = G_TRUNC [[ICMP]](s32)
+    ; CHECK: [[DEF:%[0-9]+]]:_(s64) = G_IMPLICIT_DEF
+    ; CHECK: [[SELECT:%[0-9]+]]:_(s64) = G_SELECT %cmp(s1), [[DEF]], [[DEF]]
+    ; CHECK: $x0 = COPY [[SELECT]](s64)
+    ; CHECK: RET_ReallyLR implicit $x0
+    %a:_(s32) = COPY $w0
+    %b:_(s32) = COPY $w1
+    %cmp:_(s1) = G_ICMP intpred(sgt), %a(s32), %b
+    %sel_a:_(s88) = G_IMPLICIT_DEF
+    %sel_b:_(s88) = G_IMPLICIT_DEF
+    %select:_(s88) = G_SELECT %cmp(s1), %sel_a, %sel_b
+    %trunc:_(s64) = G_TRUNC %select
+    $x0 = COPY %trunc
+    RET_ReallyLR implicit $x0