### Description
Shift the 64-bit value formed by concatenating operands a and b left or right by the amount
specified by the unsigned 32-bit value in c. Operand b holds bits 63:32 and operand a
holds bits 31:0 of the 64-bit source value. The source is shifted left or right by the clamped
or wrapped value in c. For shf.l, the most-significant 32-bits of the result are written
into d; for shf.r, the least-significant 32-bits of the result are written into d.
### Syntax
```
shf.l.mode.b32 d, a, b, c; // left shift
shf.r.mode.b32 d, a, b, c; // right shift
.mode = { .clamp, .wrap };
```
### Semantics
```
u32 n = (.mode == .clamp) ? min(c, 32) : c & 0x1f;
switch (shf.dir) { // shift concatenation of [b, a]
case shf.l: // extract 32 msbs
u32 d = (b << n) | (a >> (32-n));
case shf.r: // extract 32 lsbs
u32 d = (b << (32-n)) | (a >> n);
}
```