[RISCV][InsertVSETVLI] Avoid vmv.s.x SEW toggle if at start of block

vmv.s.x and friends that only write to the first destination element can
use any SEW greater than or equal to its original SEW, provided that
it's writing to an implicit_def operand where we can clobber the other
lanes.

We were already handling this in needVSETVLI, which meant that when
scanning the instructions from top to bottom we could detect this and
avoid the toggle:

```
	vsetivli	zero, 4, e64, mf2, ta, ma
	li	a0, 11
	vsetivli	zero, 1, e8, mf8, ta, ma
	vmv.s.x	v0, a0

->
	vsetivli	zero, 4, e64, mf2, ta, ma
	li	a0, 11
	vmv.s.x	v0, a0

```
The issue that this patch aims to solve is whenever vmv.s.x arises when
the first vector instruction in the block and doesn't have any prior
predecessor info:

```
entry_bb:
	li	a0, 11
	; No previous state here: forced to set VL/VTYPE
	vsetivli	zero, 1, e8, mf8, ta, ma
	vmv.s.x	v0, a0
	vsetivli	zero, 4, e16, mf2, ta, ma
	vmerge.vvm	v8, v9, v8, v0
```

doLocalPostpass can work backwards from bottom to top and work out if
an earlier vsetvli can be mutated to avoid a toggle. It uses
DemandedFields and getDemanded for this, which previously didn't take
into account the possibility of going to a larger SEW.

This patch adds a third option for SEW in DemandedFields, that's weaker
than demanded but stronger than not demanded, that states that it the
new SEW must be greater than or equal to the current SEW.

We can then use this option to move that vmv.s.x specific logic from
needVSETVLI into getDemanded, making it available for both phase 2 and
3, i.e. we can now mutate the earlier vsetivli going from bottom to top:

```
entry_bb:
	li	a0, 11
	; Previous vsetivli mutated: second one deleted
	vsetivli	zero, 4, e16, mf2, ta, ma
	vmv.s.x	v0, a0
	vmerge.vvm	v8, v9, v8, v0
```

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D151561