The aarch64-ldst-opt pass tries to merge two load instructions
(LDR*) to a load pair instruction (LDP*).
When merging the instructions, there is a case where one of the
loads would have to also be sign extended. In either case,
(sign extend or not), the pass needs to preserve the debug-instr-number
from the original loads to the load pair instruction to make sure debug
info
isn't lost in the case where instruction referencing is being used.
For example:
We can have something like this:
```
debugValueSubstitutions:[]
$x1 = LDRXui $x0, 1, debug-instr-number 1
DBG_INSTR_REF !13, dbg-instr-ref(1, 0), debug-location !11
$x0 = LDRXui killed $x0, 0, debug-instr-number 2
DBG_INSTR_REF !14, dbg-instr-ref(2, 0), debug-location !11
```
This would be changed to:
```
debugValueSubstitutions: []
$x0, $x1 = LDPXi $x0, 0
DBG_INSTR_REF !12, dbg-instr-ref(1, 0), debug-location !14
DBG_INSTR_REF !13, dbg-instr-ref(2, 0), debug-location !14
```
In this case, we need to create a new debug instruction number
for the `LDP` instruction, we then need to add entries into the
debugSubstitutions table to map the old instr-refs to the new ones.
After this patch, the result will be:
```
debugValueSubstitutions:
- { srcinst: 1, srcop: 0, dstinst: 3, dstop: 1, subreg: 0 }
- { srcinst: 2, srcop: 0, dstinst: 3, dstop: 0, subreg: 0 }
$x0, $x1 = LDPXi $x0, 0, debug-instr-number 3
DBG_INSTR_REF !12, dbg-instr-ref(1, 0), debug-location !14
DBG_INSTR_REF !12, dbg-instr-ref(2, 0), debug-location !14
```
However, this is not all, we also can have a case where there is a
sign-extend involved, let's look at the case:
```
debugValueSubstitutions:[]
$w1 = LDRWui $x0, 1, debug-instr-number 1
DBG_INSTR_REF !7, dbg-instr-ref(1, 0), debug-location !9
$x0 = LDRSWui $x0, 0, debug-instr-number 2
DBG_INSTR_REF !8, dbg-instr-ref(2, 0), debug-location !9
```
This will become:
```
debugValueSubstitutions:[]
$w0, $w1 = LDPWi $x0, 0
$w0 = KILL $w0, implicit-def $x0
$x0 = SBFMXri $x0, 0, 31
DBG_INSTR_REF !7, dbg-instr-ref(1, 0), debug-location !9
DBG_INSTR_REF !8, dbg-instr-ref(2, 0), debug-location !9
```
$x0 is where the final value is stored, so the sign extend (SBFMXri)
instruction contains the final value we care about we give it a new
debug-instr-number 3. Whereas, $w1 contains the final value that we care
about, therefore the LDP instruction is also given a new
debug-instr-number 4. We have to add these subsitutions to the
debugValueSubstitutions table. However, we also have to ensure that the
OpIndex that pointed to debug-instr-number 1 gets updated to 1, because
$w1 is the second operand of the LDP instruction.
The result after the patch looks like:
```
debugValueSubstitutions:
- { srcinst: 1, srcop: 0, dstinst: 4, dstop: 1, subreg: 0 }
- { srcinst: 2, srcop: 0, dstinst: 3, dstop: 0, subreg: 0 }
$w0, $w1 = LDPWi $x0, 0, debug-instr-number 4
$w0 = KILL $w0, implicit-def $x0
$x0 = SBFMXri $x0, 0, 31, debug-instr-number 3
DBG_INSTR_REF !7, dbg-instr-ref(1, 0), debug-location !9
DBG_INSTR_REF !8, dbg-instr-ref(2, 0), debug-location !9
```
This patch addresses that problem.
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