[lld][LoongArch] Support TLSDESC GD/LD to IE/LE (#123715)

Support TLSDESC to initial-exec or local-exec optimizations. Introduce a
new hook RE_LOONGARCH_RELAX_TLS_GD_TO_IE_PAGE_PC and use existing
R_RELAX_TLS_GD_TO_IE_ABS to support TLSDESC => IE, while use existing
R_RELAX_TLS_GD_TO_LE to support TLSDESC => LE.
    
In normal or medium code model, there are two forms of code sequences:
* pcalau12i  $a0, %desc_pc_hi20(sym_desc)
* addi.d     $a0, $a0, %desc_pc_lo12(sym_desc)
* ld.d       $ra, $a0, %desc_ld(sym_desc)
* jirl       $ra, $ra, %desc_call(sym_desc)
------
* pcaddi     $a0, %desc_pcrel_20(sym_desc)
* ld.d       $ra, $a0, %desc_ld(sym_desc)
* jirl       $ra, $ra, %desc_call(sym_desc)
    
Convert to IE:
* pcalau12i $a0, %ie_pc_hi20(sym_ie)
* ld.[wd]   $a0, $a0, %ie_pc_lo12(sym_ie)

Convert to LE:
* lu12i.w $a0, %le_hi20(sym_le) # le_hi20 != 0, otherwise NOP
* ori $a0 src, %le_lo12(sym_le) # le_hi20 != 0, src = $a0, otherwise src = $zero

Simplicity, whether tlsdescToIe or tlsdescToLe, we always tend to
convert the preceding instructions to NOPs, due to both forms of code
sequence (corresponding to relocation combinations:
R_LARCH_TLS_DESC_PC_HI20+R_LARCH_TLS_DESC_PC_LO12 and
R_LARCH_TLS_DESC_PCREL20_S2) have same process.
    
TODO: When relaxation enables, redundant NOPs can be removed. It will be
implemented in a future patch.
    
Note: All forms of TLSDESC code sequences should not appear interleaved
in the normal, medium or extreme code model, which compilers do not
generate and lld is unsupported. This is thanks to the guard in
PostRASchedulerList.cpp in llvm.
```
Calls are not scheduling boundaries before register allocation,
but post-ra we don't gain anything by scheduling across calls
since we don't need to worry about register pressure.
```

0 files changed, 0 insertions, 0 deletions