Too complex to add more LD/ST, a compromise is to add shift-and-add.
Replaces a pair of explicit instructions in hot-loops.
+```
+# 1.6.27 Z23-FORM
+ |0 |6 |11 |15 |16 |21 |23 |31 |
+ | PO | RT | RA | RB |sm | XO |Rc |
+```
+
```
uint_xlen_t shadd(uint_xlen_t rs1, uint_xlen_t rs2, uint8_t sh) {
sh = sh & 0x3;