This patch adds support for converting memset calls to one or more
`QC_SETWMI` instructions when beneficial. We only handle aligned memset
calls for now.
We limit a `QC_SETWMI` to 16 words or less to improve interruptibility.
So for `1-16` words we use a single `QC_SETWMI`:
`QC_SETWMI reg1, N, 0(reg2)`
For `17-32 `words we use two `QC_SETWMI's` with the first as 16 words
and the second for the remainder:
```
QC_SETWMI reg1, 16, 0(reg2)
QC_SETWMI reg1, N, 64(reg2)
```
For `33-48` words, we would like to use `(16, 16, n)`, but that means
the last QC_SETWMI needs an offset of `128` which the instruction
doesn't support. So in this case we use a length of `15` for the second
instruction and we do the rest with the third instruction.
This means the maximum number of words handled is `47` (for now):
```
QC_SETWMI R2, R0, 16, 0
QC_SETWMI R2, R0, 15, 64
QC_SETWMI R2, R0, N, 124
```
For `48` words or more, call the target independent memset.
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