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

vmv.s.x/vfmv.s.f instructions 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 arises when the vmv.s.x is
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.

A previous patch consolidated the vmv.s.x logic from needVSETVLI logic
into getDemanded, and this patch removes the gate around it so that
doLocalPostpass can now delete vsetvlis like in the scenario below:

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

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