[dev.ssa] cmd/compile: fix a few bugs for SSA for ARM
- 64x signed right shift was wrong for shift larger than 0x80000000.
- for Lsh-followed-by-Rsh, the intermediate value should be full int
width, so when it is spilled MOVW should be used.
- use RET for RetJmp, so the assembler can take case of restoring LR
for non-leaf case.
- reserve R9 in dynlink mode. R9 is used for GOT by the assembler.
Progress on SSA backend for ARM. Still not complete.
Updates #15365.
Change-Id: I3caca256b92ff7cf96469da2feaf4868a592efc5
Reviewed-on: https://go-review.googlesource.com/23793 Reviewed-by: David Chase <drchase@google.com>