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>