cmd/compile: fix confusion in generating SelectN index

Old: return the ABI register index of the result (wrong!)
New: return the index w/in sequence of result registers (right!)

Fixed bug:
genCaller0/genCaller0.go:43:9: internal compiler error: 'Caller0':
   panic during schedule while compiling Caller0:
runtime error: index out of range [10] with length 9

Updates #44816.

Change-Id: I1111e283658a2d6422986ae3d61bd95d1b9bde5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/299549
Trust: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

Author: dr2chase

src/cmd/compile/internal/ssa/expand_calls.go

@@ -424,7 +424,11 @@ func (x *expandState) rewriteSelect(leaf *Value, selector *Value, offset int64,
 				}
 				outParam := aux.abiInfo.OutParam(int(which))
 				if len(outParam.Registers) > 0 {
-					reg := int64(outParam.Registers[regOffset])
+					firstReg := uint32(0)
+					for i := 0; i < int(which); i++ {
+						firstReg += uint32(len(aux.abiInfo.OutParam(i).Registers))
+					}
+					reg := int64(regOffset + Abi1RO(firstReg))
 					if leaf.Block == call.Block {
 						leaf.reset(OpSelectN)
 						leaf.SetArgs1(call0)